devel/cmake: fix build on GCC architectures
Manually define __BSD_VISIBLE to make timespec_get() available:
In file included from /usr/local/lib/gcc9/include/c++/chrono:41,
from /wrkdirs/usr/ports/devel/cmake/work/cmake-3.18.1/Source/cmDuration.h:5, from /wrkdirs/usr/ports/devel/cmake/work/cmake-3.18.1/Source/cmSystemTools.h:19, from /wrkdirs/usr/ports/devel/cmake/work/cmake-3.18.1/Source/cmSystemTools.cxx:13:
/usr/local/lib/gcc9/include/c++/ctime:80:11: error: '::timespec_get' has not been declared
80 | using ::timespec_get; | ^~~~~~~~~~~~
PR: 248726