Allow compiling with clang and using clang-tidy analyzer

[Siergiej Riaguzow]

clang-tidy analyzer is known to be one of the best linters/static code analyzers out there. It is enough to enable ClangCodeModel plugin in QtCreator for the xpad project to see a lot of warnings.

If xpad were using cmake build system, adding an option to build with clang and another one to run clang-tidy with different tests would be easier (at least for me as I know how to do that).

With autogen.sh/configure and autoreconf stuff I have no idea how to add a possibility to:

./configure --with-clang
./configure --with-clang --with-clang-tidy

or similar.

This is a very low priority just to find out how to integrate this. I've read that they are serious about recompiling everything in FreeBSD with clang: https://wiki.freebsd.org/BuildingFreeBSDWithClang. Why not add a switch for them to also allow to compile xpad with clang?

Additionaly clang-tidy static analyzer will help to find bugs. I think this is a really low priority and if anyone want to take it from me - please do. Assigning to myself just to not loose this.