Merge #10971: build: fix missing sse42 in depends builds

9baca41 build: always attempt to enable targeted sse42 cxxflags (Cory Fields)

Pull request description:

  For depends builds without this, configure thinks that the user has overridden CXXFLAGS manually, when really they've just been set by the config.site.

  The effect is that warnings and extra cxxflags (sse4.2 for example) were not being added.

Tree-SHA512: 9fd615ad0e926bd9d6b541ffcf7fc555e2147e8761f57ff3b5fb5d196c9cef0f26aa99681ff72db8c83c0f9a7ed91f4253f46bab09f2c835044b68047358fa47

configure.ac

@@ -258,14 +258,13 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[CXXFLAGS="$CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wdeprecated-register],[CXXFLAGS="$CXXFLAGS -Wno-deprecated-register"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough],[CXXFLAGS="$CXXFLAGS -Wno-implicit-fallthrough"],,[[$CXXFLAG_WERROR]])
-
-  # Check for optional instruction set support. Enabling these does _not_ imply that all code will
-  # be compiled with them, rather that specific objects/libs may use them after checking for runtime
-  # compatibility.
-  AX_CHECK_COMPILE_FLAG([-msse4.2],[[SSE42_CXXFLAGS="-msse4.2"]],,[[$CXXFLAG_WERROR]])
-
 fi
 
+# Check for optional instruction set support. Enabling these does _not_ imply that all code will
+# be compiled with them, rather that specific objects/libs may use them after checking for runtime
+# compatibility.
+AX_CHECK_COMPILE_FLAG([-msse4.2],[[SSE42_CXXFLAGS="-msse4.2"]],,[[$CXXFLAG_WERROR]])
+
 TEMP_CXXFLAGS="$CXXFLAGS"
 CXXFLAGS="$CXXFLAGS $SSE42_CXXFLAGS"
 AC_MSG_CHECKING(for assembler crc32 support)