--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -446,18 +446,22 @@ if test "$GNU_CC"; then
         # symbols for shared objects in some cases.
         if test -z "$MOZ_NO_WLZDEFS"; then
             # Don't allow undefined symbols in libraries
             DSO_LDOPTS="$DSO_LDOPTS -Wl,-z,defs"
         fi
     fi
     WARNINGS_AS_ERRORS='-Werror'
     DSO_CFLAGS=''
-    DSO_PIC_CFLAGS='-fPIC'
-    ASFLAGS="$ASFLAGS -fPIC"
+
+    if test "$OS_ARCH" != "WINNT"; then
+        DSO_PIC_CFLAGS='-fPIC'
+        ASFLAGS="$ASFLAGS -fPIC"
+    fi
+
     AC_MSG_CHECKING([for --noexecstack option to as])
     _SAVE_CFLAGS=$CFLAGS
     CFLAGS="$CFLAGS -Wa,--noexecstack"
     AC_TRY_COMPILE(,,AC_MSG_RESULT([yes])
                      [ASFLAGS="$ASFLAGS -Wa,--noexecstack"],
                      AC_MSG_RESULT([no]))
     CFLAGS=$_SAVE_CFLAGS
     AC_MSG_CHECKING([for -z noexecstack option to ld])
@@ -643,24 +647,30 @@ case "$target" in
         CXX="$CXX -mwindows"
         CPP="$CPP -mwindows"
         CFLAGS="$CFLAGS -mms-bitfields"
         CXXFLAGS="$CXXFLAGS -mms-bitfields"
         DSO_LDOPTS='-shared'
         MKSHLIB='$(CXX) $(DSO_LDOPTS) -o $@'
         MKCSHLIB='$(CC) $(DSO_LDOPTS) -o $@'
         RC='$(WINDRES)'
-        # Use static libgcc and libstdc++
-        LDFLAGS="$LDFLAGS -static"
         # Use temp file for windres (bug 213281)
         RCFLAGS='-O coff --use-temp-file'
         # mingw doesn't require kernel32, user32, and advapi32 explicitly
         LIBS="$LIBS -lusp10 -lgdi32 -lwinmm -lwsock32"
         MOZ_FIX_LINK_PATHS=
 
+        if test -z "$CLANG_CC"; then
+            # Use static libgcc and libstdc++
+            LDFLAGS="$LDFLAGS -static"
+        else
+            # Silence problematic clang warnings
+            CXXFLAGS="$CXXFLAGS -Wno-incompatible-ms-struct"
+        fi
+
         MOZ_OPTIMIZE_FLAGS="-O2"
 
         WIN32_CONSOLE_EXE_LDFLAGS=-mconsole
         WIN32_GUI_EXE_LDFLAGS=-mwindows
     else
         TARGET_COMPILER_ABI=msvc
         HOST_CC='$(CC)'
         HOST_CXX='$(CXX)'
@@ -805,17 +815,17 @@ case "$target" in
     MOZ_USER_DIR="Mozilla"
 
     case "$host_os" in
     cygwin*|msvc*|mks*)
         AC_MSG_ERROR([Using a Cygwin build environment is unsupported. Configure cannot check for presence of necessary headers. Please upgrade to MozillaBuild; see https://developer.mozilla.org/en/Windows_Build_Prerequisites.])
         ;;
     esac
 
-    if test -n "$GNU_CC"; then
+    if test -n "$GNU_CC" -a -z "$CLANG_CC"; then
         CFLAGS="$CFLAGS -fno-keep-inline-dllexport"
         CXXFLAGS="$CXXFLAGS -fno-keep-inline-dllexport"
     fi
 
     case "$target" in
     i*86-*)
         if test -n "$GNU_CC"; then
             CFLAGS="$CFLAGS -mstackrealign"

--- a/old-configure.in
+++ b/old-configure.in
@@ -531,18 +531,22 @@ if test "$GNU_CC"; then
     SSE2_FLAGS="-msse2"
     SSSE3_FLAGS="-mssse3"
     # FIXME: Let us build with strict aliasing. bug 414641.
     CFLAGS="$CFLAGS -fno-strict-aliasing"
     MKSHLIB='$(CXX) $(COMPUTED_CXX_LDFLAGS) $(PGO_CFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -Wl,-h,$(DSO_SONAME) -o $@'
     MKCSHLIB='$(CC) $(COMPUTED_C_LDFLAGS) $(PGO_CFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -Wl,-h,$(DSO_SONAME) -o $@'
     WARNINGS_AS_ERRORS='-Werror'
     DSO_CFLAGS=''
-    DSO_PIC_CFLAGS='-fPIC'
-    ASFLAGS="$ASFLAGS -fPIC"
+
+    if test "$OS_ARCH" != "WINNT" -o -z "$CLANG_CC"; then
+        DSO_PIC_CFLAGS='-fPIC'
+        ASFLAGS="$ASFLAGS -fPIC"
+    fi
+
     AC_MSG_CHECKING([for --noexecstack option to as])
     _SAVE_CFLAGS=$CFLAGS
     CFLAGS="$CFLAGS -Wa,--noexecstack"
     AC_TRY_COMPILE(,,AC_MSG_RESULT([yes])
                      [ASFLAGS="$ASFLAGS -Wa,--noexecstack"],
                      AC_MSG_RESULT([no]))
     CFLAGS=$_SAVE_CFLAGS
     AC_MSG_CHECKING([for -z noexecstack option to ld])
@@ -851,34 +855,40 @@ case "$target" in
         CXX="$CXX -mwindows"
         CPP="$CPP -mwindows"
         CFLAGS="$CFLAGS -mms-bitfields"
         CXXFLAGS="$CXXFLAGS -mms-bitfields"
         DSO_LDOPTS='-shared'
         MKSHLIB='$(CXX) $(DSO_LDOPTS) -o $@'
         MKCSHLIB='$(CC) $(DSO_LDOPTS) -o $@'
         RC='$(WINDRES)'
-        # Use static libgcc and libstdc++
-        LDFLAGS="$LDFLAGS -static"
         # Use temp file for windres (bug 213281)
         RCFLAGS='-O coff --use-temp-file'
         # mingw doesn't require kernel32, user32, and advapi32 explicitly
         LIBS="$LIBS -luuid -lusp10 -lgdi32 -lwinmm -lwsock32 -luserenv -lsecur32"
         MOZ_FIX_LINK_PATHS=
 
         MOZ_OPTIMIZE_FLAGS="-O1"
 
         WIN32_CONSOLE_EXE_LDFLAGS=-mconsole
         WIN32_GUI_EXE_LDFLAGS=-mwindows
 
-        # GCC/binutils can't link to a function if we try to include dllexport function
-        # in the same library as dllimport caller. To work around it, we build NSPR
-        # and NSS with -mnop-fun-dllimport flag. The drawback of this solution is that
-        # function thunks need to be generated for cross-DLL calls.
-        MOZ_FOLD_LIBS_FLAGS="-mnop-fun-dllimport"
+        if test -z "$CLANG_CC"; then
+            # Use static libgcc and libstdc++
+            LDFLAGS="$LDFLAGS -static"
+
+            # GCC/binutils can't link to a function if we try to include dllexport function
+            # in the same library as dllimport caller. To work around it, we build NSPR
+            # and NSS with -mnop-fun-dllimport flag. The drawback of this solution is that
+            # function thunks need to be generated for cross-DLL calls.
+            MOZ_FOLD_LIBS_FLAGS="-mnop-fun-dllimport"
+        else
+            # Silence problematic clang warnings
+            CXXFLAGS="$CXXFLAGS -Wno-incompatible-ms-struct"
+        fi
     else
         TARGET_COMPILER_ABI=msvc
         if test "$AS_BIN"; then
             AS="$(basename "$AS_BIN")"
         fi
         AR='lib'
         AR_FLAGS='-NOLOGO -OUT:$@'
         AR_EXTRACT=
@@ -1049,17 +1059,17 @@ case "$target" in
     MOZ_USER_DIR="Mozilla"
 
     case "$host_os" in
     cygwin*|msvc*|mks*)
         AC_MSG_ERROR([Using a Cygwin build environment is unsupported. Configure cannot check for presence of necessary headers. Please upgrade to MozillaBuild; see https://developer.mozilla.org/en/Windows_Build_Prerequisites.])
         ;;
     esac
 
-    if test -n "$GNU_CC"; then
+    if test -n "$GNU_CC" -a -z "$CLANG_CC"; then
         CFLAGS="$CFLAGS -fno-keep-inline-dllexport"
         CXXFLAGS="$CXXFLAGS -fno-keep-inline-dllexport"
     fi
 
     case "$target" in
     i*86-*)
         if test -n "$GNU_CC"; then
             CFLAGS="$CFLAGS -mstackrealign"