Fix the cmake Sphinx find script.

The cmake sphinx find script version parsing assumes a format of 1.2.3, but sphinx version can be 1.2b3 as well, so parse those correctly as well.
Create and install the pkg-config file jansson.pc for CMake.

Use the same jansson.pc.in file as the ./configure script does to generate the jansson.pc file for pkg-config. Also make an install rule for this.
Merge pull request #115 from JoakimSoderberg/cmake_pkgconfig

Cmake pkgconfig