lua.pc.in: add INSTALL_{L,C}MOD variables

Note:

Most distributions are defining INSTALL_LMOD and INSTALL_CMOD within their
packaged pkg-config file of lua, because both are commonly used by third-party
Makefiles for figuring out where to install lua modules through pkg-config.

INSTALL_LMOD defines the path where to install pure lua modules
INSTALL_CMOD defines the path where to install compiled lua modules
Merge pull request #8 from morfoh/lua-5.1.4

lua.pc.in: add INSTALL_{L,C}MOD variables