Ignore build files

Ignore various files created by autotools, so they don't clutter the
"git status" output.

.gitignore

@@ -12,12 +12,19 @@ config.sub
 configure
 contrib/WindowMaker.spec
 install-sh
+INSTALL
 libtool
 ltmain.sh
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
 missing
 mkinstalldirs
 src/config.h
 src/config.h.in
+src/config.h.in~
 src/stamp-h1
 src/wconfig.h
 wmlib/wmlib.pc