Parallelization bug #23 and cmake_constant_file

[MartKro]

• Fixed the parallelization bug #23 which prevents possible parallelization due to the introduced mutex. Now the mutex is also protected using __thread but since it is implementation dependent the flag can be easily disabled
• Changed the introduction of constants from CMAKE to an extra file cmake_constants.h.in

[codedust]

This doesn't introduce thread local mutexes when using OpenMP, right? Could you also declare the mutex threadprivate if OpenMP is enabled? (like it's done here with the context object: https://github.com/relic-toolkit/relic/blob/c25edf6520e5e4b39663f0d0594db7186440be5c/src/relic_core.c#L95)