v4: Fix FTBS on undefined clock_gettime()
Some platforms/libc's (eg. SunOS) explicitly need the POSIX level defined in order to define clock_gettime().
changes v4:
- drop summary() message on SunOS
- use == instead of contains on check for host platform.
changes v3:
- using host_machine.system() instead of compile test for OS detection
changes v2:
- setting '_EXTENSIONS_' globally when on Solaris/Illumos
Edited by Enrico Weigelt, metux IT consult .