--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -564,19 +564,19 @@ dnl In at least glibc-2.25, _FORTIFY_SOU
 dnl with optimization (Bug 1417452)
 
 dnl Note that in the top-level old-configure.in, we don't enable
 dnl FORTIFY_SOURCE on Android. But in js/ we *can* enable it on
 dnl Android, so we do.
 
 if test -n "$MOZ_OPTIMIZE"; then
    if test "$GNU_CC" -o -n "${CLANG_CC}${CLANG_CL}"; then
-      CFLAGS="$CFLAGS -D_FORTIFY_SOURCE=2"
-      CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
-      CXXFLAGS="$CXXFLAGS -D_FORTIFY_SOURCE=2"
+      CFLAGS="$CFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
+      CPPFLAGS="$CPPFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
+      CXXFLAGS="$CXXFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
    fi
 fi
 
 dnl ========================================================
 dnl System overrides of the defaults for target
 dnl ========================================================
 
 case "$target" in

--- a/old-configure.in
+++ b/old-configure.in
@@ -513,19 +513,19 @@ if test -n "$MOZ_OPTIMIZE"; then
    if test "$GNU_CC" -o -n "${CLANG_CC}${CLANG_CL}"; then
       case $OS_TARGET in
          Android)
             dnl FORTIFY_SOURCE is not supported on Android on the
             dnl top-level old-configure.in at this time.
             dnl See Bug 1415595
             ;;
          *)
-            CFLAGS="$CFLAGS -D_FORTIFY_SOURCE=2"
-            CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
-            CXXFLAGS="$CXXFLAGS -D_FORTIFY_SOURCE=2"
+            CFLAGS="$CFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
+            CPPFLAGS="$CPPFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
+            CXXFLAGS="$CXXFLAGS -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"
             ;;
       esac
    fi
 fi
 
 dnl ========================================================
 dnl GNU specific defaults
 dnl ========================================================