--- a/build/autoconf/compiler-opts.m4
+++ b/build/autoconf/compiler-opts.m4
@@ -16,18 +16,16 @@ dnl set DEVELOPER_OPTIONS early; MOZ_DEF
       DEVELOPER_OPTIONS=,
       DEVELOPER_OPTIONS=1)
 
 dnl Default to MSVC for win32 and gcc-4.2 for darwin
 dnl ==============================================================
 if test -z "$CROSS_COMPILE"; then
 case "$target" in
 *-mingw*)
-    if test -z "$CC"; then CC=cl; fi
-    if test -z "$CXX"; then CXX=cl; fi
     if test -z "$CPP"; then CPP="$CC -E -nologo"; fi
     if test -z "$CXXCPP"; then CXXCPP="$CXX -TP -E -nologo"; ac_cv_prog_CXXCPP="$CXXCPP"; fi
     if test -z "$AS"; then
         case "${target_cpu}" in
         i*86)
             AS=ml;
             ;;
         x86_64)
@@ -37,33 +35,16 @@ case "$target" in
     fi
     if test -z "$MIDL"; then MIDL=midl; fi
 
     # need override this flag since we don't use $(LDFLAGS) for this.
     if test -z "$HOST_LDFLAGS" ; then
         HOST_LDFLAGS=" "
     fi
     ;;
-*-darwin*)
-    # GCC on darwin is based on gcc 4.2 and we don't support it anymore.
-    if test -z "$CC"; then
-        MOZ_PATH_PROGS(CC, clang)
-    fi
-    if test -z "$CXX"; then
-        MOZ_PATH_PROGS(CXX, clang++)
-    fi
-    IS_GCC=$($CC -v 2>&1 | grep gcc)
-    if test -n "$IS_GCC"
-    then
-      echo gcc is known to be broken on OS X, please use clang.
-      echo see http://developer.mozilla.org/en-US/docs/Developer_Guide/Build_Instructions/Mac_OS_X_Prerequisites
-      echo for more information.
-      exit 1
-    fi
-    ;;
 esac
 fi
 ])
 
 dnl ============================================================================
 dnl C++ rtti
 dnl We don't use it in the code, but it can be usefull for debugging, so give
 dnl the user the option of enabling it.

--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -976,20 +976,16 @@ case "$target" in
     case "$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
 
     case "$target" in
     i*86-*)
-        if test "$HAVE_64BIT_BUILD"; then
-            AC_MSG_ERROR([You are targeting i386 but using the 64-bit compiler.])
-        fi
-
         if test -n "$GNU_CC"; then
             CFLAGS="$CFLAGS -mstackrealign -fno-keep-inline-dllexport"
             CXXFLAGS="$CXXFLAGS -mstackrealign -fno-keep-inline-dllexport"
             LDFLAGS="$LDFLAGS -Wl,--large-address-aware"
         else
             DSO_LDOPTS="$DSO_LDOPTS -MACHINE:X86"
             LDFLAGS="$LDFLAGS -SAFESEH"
         fi

--- a/old-configure.in
+++ b/old-configure.in
@@ -1337,35 +1337,28 @@ case "$target" in
     case "$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
 
     case "$target" in
     i*86-*)
-        if test "$HAVE_64BIT_BUILD" -a -n "$COMPILE_ENVIRONMENT"; then
-            AC_MSG_ERROR([You are targeting i386 but using the 64-bit compiler.])
-        fi
-
         if test -n "$GNU_CC"; then
             CFLAGS="$CFLAGS -mstackrealign -fno-keep-inline-dllexport"
             CXXFLAGS="$CXXFLAGS -mstackrealign -fno-keep-inline-dllexport"
             LDFLAGS="$LDFLAGS -Wl,--enable-stdcall-fixup -Wl,--large-address-aware"
         else
             DSO_LDOPTS="$DSO_LDOPTS -MACHINE:X86"
             LDFLAGS="$LDFLAGS -SAFESEH"
         fi
 
         AC_DEFINE(_X86_)
         ;;
     x86_64-*)
-        if test -z "$HAVE_64BIT_BUILD" -a -n "$COMPILE_ENVIRONMENT"; then
-            AC_MSG_ERROR([You are targeting 64-bit but using the 32-bit compiler.])
-        fi
         if test -n "$_WIN32_MSVC"; then
             DSO_LDOPTS="$DSO_LDOPTS -MACHINE:X64"
         fi
         AC_DEFINE(_AMD64_)
         ;;
     *)
         AC_DEFINE(_CPU_ARCH_NOT_DEFINED)
         ;;