Enable -Werror in GTK builds (we already have -Wall, so more GCCisms aren't

_that_ evil).

[originally from svn r2667]

mkfiles.pl

@@ -539,7 +539,7 @@ print
 "# TOOLPATH = /opt/gcc/bin\n".
 "CC = \$(TOOLPATH)cc\n".
 "\n".
-&splitline("CFLAGS = -Wall -g -I. -I.. -I../charset `gtk-config --cflags`")."\n".
+&splitline("CFLAGS = -Wall -Werror -g -I. -I.. -I../charset `gtk-config --cflags`")."\n".
 "XLDFLAGS = `gtk-config --libs`\n".
 "ULDFLAGS =#\n".
 "INSTALL=install\n",