Better check for MSVC

Change-Id: Ida9197e662f4f13baf834e54e2ff4f7dff65160f
Reviewed-on: https://gerrit.libreoffice.org/60312
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -6413,7 +6413,7 @@ AC_LANG_PUSH([C++])
 save_CXXFLAGS=$CXXFLAGS
 CXXFLAGS="$CXXFLAGS $CXXFLAGS_CXX11"
 dnl Unknown attributes must be ignored by compilers, but they do emit warnings about them:
-if test "$_os" = WINNT; then
+if test "$COM" = MSC; then
     CXXFLAGS="$CXXFLAGS /WX"
 else
     CXXFLAGS="$CXXFLAGS -Werror"