Add basic CMake support

[flandr]

Only supports building the main library target (static and dynamic).

Refs #257

[bnoordhuis]

Thanks. The changes mostly look good to me but I'm curious why you are using set() for everything. I'm porting node's build system to cmake (tree) and the CMakeLists.txt for http_parser essentially consists out of add_library(http_parser http_parser.c) plus a target_include_directories() to export the include path.

[flandr]

This is just a specialization of a default template I keep around for cmake builds. Some of it is not specifically necessary.

If you are going to do a full port to cmake for node, feel free to ignore this pull request.