Add "make install" support for distros with no INSTALL_{C,L}MOD vars
Add "make install" support for distros with no INSTALL_{C,L}MOD vars

Some packagers provide these helpful variables in their pkg-config
files. For those that don't, we construct them from the "prefix"
and "libdir" variables instead.

craigbarnes/lua-gumbo,craigbarnes/lua-gumbo,craigbarnes/lua-gumbo