Added some static asserts.

[HarveyHunt]

Opinions on this would be useful:

• Do these asserts belong in the config file?
• Shall I add asserts for any of the other #defined config variables?
• Shall I add asserts to check things within the arrays in config.h, such as wss?

_Static_assert compiles fine under both GCC and Clang, so we don't need to worry about that. :-)

[anubhavcodes]

1) As the config.h options will be modified by users of howm, putting static asserts in config.h seems to be the most appropriate idea.

2) Static asserts can also be added to check the things in the arrays of config.h, but since they won't be modified much by the users, using a test case mechanisms (unit tests or regression tests for arrays) seems wise here. We can have a look at cunit and other packages for testing available for C. But using static asserts should also work for this use case. This is just my opinion.

[HarveyHunt]

1. Okay, I agree with that so the asserts will stay where they are.
2. I am rather scared about adding unit tests to howm, I think we will end up with lots of code and it will be hard to ensure code works correctly with the X server- a big part of howm. I think I will add some more static asserts to config.h, but I don't want run time tests really, as people aren't going to see the output of howm.

[anubhavcodes]

Okay, then static asserts for checking the arrays sounds good!

[HarveyHunt]

All done, time to merge. :+1: