waf, parser: Able to specify Lua version to link against

Yep, just use --lua=VERSION when configuring. Takes the same version string as pkg-config does. pkg-config will be asked for the include path, which is then used by the preprocessor to include the lua headers. I can't ask waf to give me the cflags of a uselib_store, so it's the only way. Also, another hack is using "path+headerfile" instead of <path+headerfile> to include headers. Waf can't unquote stuff when making config.h. I hope no one will complain if anything goes wrong. It works fine on my machine.

Important point is you can now have luajit handling your spawn and git add -Aexecgit add -A function. It's so ridiculously unbelievably fast it's funny. Holy shit it's fast. I still need to figure out which exact version to put as default. Everyone has 5.1, few have 5.2 and few have jit. But damn is luajit fast.