Do not treat warnings as errors in gtk4 port

The port is in early stages and there are still many deprecated usages
that should be ported rather than silenced (as in the gtk3 port).

Author: akurtakov

bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/make_linux.mak

@@ -111,7 +111,9 @@ LFLAGS = -shared -fPIC ${SWT_LFLAGS}
 
 # Treat all warnings as errors. If your new code produces a warning, please
 # take time to properly understand and fix/silence it as necessary.
+ifeq ($(GTK_VERSION), 3.0)
 CFLAGS += -Werror
+endif
 
 ifndef NO_STRIP
 	# -s = Remove all symbol table and relocation information from the executable.