Add flag to build compiler options with G++ on macOS.

Building with G++ on Darwin breaks when either AVX, AVX2, or SSE4.1
compiler option is set, unless G++ is actually CLANG.

This commit allows to build with G++, by asking G++ to delegate assembly
to the clang integrated assembler, instead of the GNU one.

configure.ac

@@ -131,6 +131,21 @@ if $sse41; then
     AM_CONDITIONAL([SSE41_OPT], true)
 fi
 
+# Add platform-specific flags for the supported compiler options.
+if $avx -o $avx2 -o $sse41; then
+  case "${host_os}" in
+    *darwin* | *-macos10*)
+       if test -z "$CLANG"; then
+	 echo "MODIFYING FLAGS"
+         # When using AVX, AVX2, or SSE4.1:
+         # Must tell AS to use clang integrated assembler,
+         # instead of the GNU based system assembler.
+         CXXFLAGS="$CXXFLAGS -Wa,-q"
+       fi
+      ;;
+  esac
+fi
+
 includedir="${includedir}/tesseract"
 
 AC_ARG_WITH([extra-includes],