Add isinf() fix to lua_cmsgpack.c

Commit https://github.com/antirez/lua-cmsgpack/commit/1852b23068efeda00d9ead61f1e721ecfb667af5 broke cmsgpack building on Solaris.

There was a comment saying "In the lua-cmsgpack there was this that I fixed a few days ago since 
it broken build in Solaris: The above should not pass reviews."

But, the new one shouldn't pass reviews either since it also broke.  :)

We can use the workaround from lua_cjson to implement `isinf()` here too.