Support building with CMake

This commit adds five CMakeLists.txt files to support building fxite
using CMake [http://cmake.org/].

The changeset is non-invasive, as none of the existing files is modified.

Just like the autotools-based build system, the CMake-based one supports a
boolean option named WITH_SYSTEM_LUA to build against the installed Lua
library. The option is OFF by default.