Add .gitignore file

.gitignore

+Makefile
+Makefile.in
+.deps
+.libs
+.msg
+*.lo
+*.la
+*.a
+*.o
+*~
+aclocal.m4
+autom4te.cache
+compile
+config.guess
+config.log
+config.status
+config.sub
+configure
+depcomp
+install-sh
+libtool
+ltmain.sh
+missing
+pixman.pc
+stamp-h?
+config.h
+config.h.in
+.*.swp
+test/composite-test
+test/gradient-test