Move all the cmake commands for finding dependencies into a separate file

[piponazo]

In this Pull Request I continue with the cleaning/refactoring step of the CMake code.

This time I just move all the CMake commands that are used to find the dependencies into a new file. I also took this opportunity to do other improvements:

• FindExpat.cmake has been removed. It is already included in CMake.
• Remove CMAKE_REQUIRED_INCLUDES that was set but never used.
• Remove some unneeded checks (already done internally in FindExpat)
• Only try to find MSGFMT if we want to use it.

[clanmills]

Thank You for doing this @piponazo

I'm working hard on the CMake/static/external library stuff at the moment. I'll pull your code into master in a couple of hours when I've finished what I'm doing. I'm working on the same files that you have modified. I hope the weather's as beautiful for you as it is for us in England today.