Abstract platform specific code

[JayFoxRox]

• constructors are GNU specific I have a solution for this locally which at least also works with MSVC
• Aligned memory allocations use platform specific code I have a solution for this locally which also works on Windows
• unistd.h is included, probably only for usleep which could be replaced by thrd_sleep
• sys/time.h is included by c11threads, for nanosleep and timespec
• pthreads should be a requirement if the compiler does not provide C11 threads

Time related functions should be dropped from c11threads, with the exception of thrd_sleep, which should use a solution like https://gist.github.com/Youka/4153f12cf2e17a77314c A custom timespec struct has to be provided for MSVC. c11threads should be updated to the upstream implementation first.

[JayFoxRox]

This has been done in #75 and followups #77 , #79 and #82 .