Bug 1272358. Add support for SSSE3 flags to configure. r=glandium

Adds a SSSE3_FLAGS variable that we can use for building source that
requires SSSE3

old-configure.in

@@ -865,6 +865,7 @@ if test "$GNU_CC"; then
     MMX_FLAGS="-mmmx"
     SSE_FLAGS="-msse"
     SSE2_FLAGS="-msse2"
+    SSSE3_FLAGS="-mssse3"
     # FIXME: Let us build with strict aliasing. bug 414641.
     CFLAGS="$CFLAGS -fno-strict-aliasing"
     MKSHLIB='$(CXX) $(CXXFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -Wl,-h,$(DSO_SONAME) -o $@'
@@ -1382,6 +1383,9 @@ case "$target" in
             CXXFLAGS="$CXXFLAGS -arch:IA32"
             SSE_FLAGS="-arch:SSE"
             SSE2_FLAGS="-arch:SSE2"
+            dnl MSVC allows the use of intrinsics without any flags
+            dnl and doesn't have a separate arch for SSSE3
+            SSSE3_FLAGS="-arch:SSE2"
         fi
         dnl VS2013+ requires -FS when parallel building by make -jN.
         dnl If nothing, compiler sometimes causes C1041 error.
@@ -1601,6 +1605,7 @@ esac
 AC_SUBST_LIST(MMX_FLAGS)
 AC_SUBST_LIST(SSE_FLAGS)
 AC_SUBST_LIST(SSE2_FLAGS)
+AC_SUBST_LIST(SSSE3_FLAGS)
 
 AC_SUBST(MOZ_LINKER)
 if test -n "$MOZ_LINKER"; then