add support for pkg-config for lua

Allow select what pkg-config module to use with UWSGICONFIG_LUAPC but
let the old LUAINC/LUALIB and LUALIBPATH override the pkg-config
for backwards compatibility.
add support for lua5.2

The default is still lua5.1 but 5.2 can be picked at compile time
but setting UWSGICONFIG_LUAPC=lua5.2