Check for missing header files

[Pan7]

More support for non Unix-like systems.

[pooler]

What systems (requiring the patch) have you tested this on?

[Pan7]

Trying to compile cpuminer with msvc on a 32 bit machine.

[pooler]

Can you build cpuminer on that system with just this patch? If sys/time.h isn't available, where is gettimeofday() declared?

[Pan7]

Didn't find a good solution for gettimeofday yet aside from using something like f4exb/sdrangel@f61d1c3#diff-8fb557527cc23eb19b0d8adcfb3294dfe3735cb89f7bc04a2d9ae865301c14fe
There is also a couple of small things missing like:
#ifdef _MSC_VER
#define strncasecmp _strnicmp
#endif
Not sure if this is the best sullution or where to add it.

[pooler]

Compatibility macros should probably go in compat.h.