Embedded platforms currently not supported in libOSDP

[Sympatron]

This commit made building libOSDP on anything other than Linux or Windows impossible. Before that I solved it by implementing gettimeofday myself, but I never really liked this solution.

I would suggest not to rely on gettimeofday in this case, but to expect usec_now to be provided by the user instead. This would make compiling libOSDP for embedded platforms way easier.

What do you think?