Add make clang-format

Makefile.am

@@ -28,3 +28,15 @@ dist_doc_DATA = README.rst
 
 EXTRA_DIST = nghttpx.conf.sample proxy.pac.sample android-config android-make \
 	Dockerfile.android
+
+.PONY: clang-format
+
+# Format source files using clang-format.  Don't format source files
+# under third-party directory since we are not responsible for thier
+# coding style.
+clang-format:
+	CLANGFORMAT=`git config --get clangformat.binary`; \
+	test -z $${CLANGFORMAT} && CLANGFORMAT="clang-format"; \
+	$${CLANGFORMAT} -i lib/*.{c,h} lib/includes/nghttp2/*.h \
+	src/*.{c,cc,h} src/includes/nghttp2/*.h examples/*.{c,cc} \
+	tests/*.{c,h}