In our CMakeLists.txt, we have to check multiple functions in order to
determine if we have to use our own or whether we can use the
platform-provided one. For two of these functions, namely `regcomp_l()`
and `futimens`, the defined macro is actually used inside of the header
file "src/unix/posix.h". As such, these macros are not only required by
the library, but also by our test suite, which is makes use of internal
headers.

To prepare for the CMakeLists.txt split, move these two defines inside
of the "features.h" header.