Register gmp with pkg-config

[Irina Gossmann]

Is there any reason for not including a .pc file with gmp? Since it's not
registered with pkg-config, code depending on it won't build without some
addition fiddling (I'm on Linux and OSX, not sure if/how it works
differently on other platforms).

If you have a way of accepting contributions from outside, I can contribute
a pkg-config file. Couldn't find a howto for contributors on the website,
would've sent a patch directly otherwise.

 -- Irina