compile GTK+ test program using C compiler, not C++ one, otherwise it always fails as it uses undeclared functions

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38799 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

configure.in

@@ -2820,6 +2820,9 @@ if test "$wxUSE_GUI" = "yes"; then
     fi
 
     if test "$wxUSE_GTK" = 1; then
+        dnl GTK+ test program must be compiled with C compiler
+        AC_LANG_PUSH(C)
+
         AC_MSG_CHECKING([for GTK+ version])
 
         gtk_version_cached=1
@@ -2916,6 +2919,8 @@ libraries returned by 'pkg-config gtk+-2.0 --libs' or 'gtk-config
                     ;;
         esac
 
+        AC_LANG_POP
+
         if test "$WXGTK20" = 1; then
             save_CFLAGS="$CFLAGS"
             save_LIBS="$LIBS"