Fix libdir detection

This patch fix installation issue at least on Debian and Ubuntu.

In lua5.1.pc delivered in debian package, variable to be used to install lua
binary module is INSTALL_CMOD:

 # The following are intended to be used via "pkg-config --variable".
 # Install paths for Lua modules.  For example, if a package wants to install
 # Lua source modules to the /usr/local tree, call pkg-config with
 # "--define-variable=prefix=/usr/local" and "--variable=INSTALL_LMOD".
 INSTALL_LMOD=${prefix}/share/lua/${major_version}
 INSTALL_CMOD=${prefix}/lib/${deb_host_multiarch}/lua/${major_version}