Fix pkg config

These are the major issues dealt with in these mods (others are in the commit messages):

*  configure derived values weren't used everywhere they could be, leading to e.g. paths in lua.pc being out of sync  with those in luaconf.h

* LUA_CDIR was hardwired as /usr/lib or /usr/lib64, regardless of where Lua was installed

Some enhancements:

* lib64 was automatically used for 64 bit systems; this may not be appropriate for all installations.  The new switch (--enable-lib64) makes that optional. The default retains the existing behavior.
* Some installations insert the lua version into the include path and the library name (similar, but not quite the same as what the libtool -release option provides) to allow installation of multiple versions of lua.  The new switch --enable-versioned-filenames  provides that behavior.