Get full version of gcc

Recent gcc versions return a short version with -dumpversion, e.g. 9.

In this case, use -dumpfullversion, which gives something like 9.1.0.

This makes the gcc 9 detection work properly.

config/lyxinclude.m4

@@ -381,6 +381,10 @@ if test x$GXX = xyes; then
   if test x$CLANG = xno; then
     dnl Useful for global version info
     gxx_version=`${CXX} -dumpversion`
+    case $gxx_version in
+	*.*) ;;
+	*) gxx_version=`${CXX} -dumpfullversion` ;;
+    esac
     CXX_VERSION="($gxx_version)"
   else
     gxx_version=clang-$clang_version