WkPlatform : improve Posix check

[asmodehn]

WkPlatform.cmake now looks for unistd.h and if found , the WkPlatform.h header then define the WK_PLATFORM_POSIX preprocessor definition.
However POSIX requires more than unistd.h
We need to improve the platform detection code, at least for posix checks inside the cmake script. A good start is to have a look at boost/config/posix_features.hpp, which should contain what is needed by the POSIX standard...
In general the platform detection WkCMake should be improved and be more standardized if ever possible, but this task concerns posix only, which is a major target platform.