build: always attempt to enable targeted sse42 cxxflags

This avoids a counter-intuitive drop in performance when manually adjusting the
flags.

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]])
+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]])
 
-fi
-
 TEMP_CXXFLAGS="$CXXFLAGS"
 CXXFLAGS="$CXXFLAGS $SSE42_CXXFLAGS"
 AC_MSG_CHECKING(for assembler crc32 support)