x11-wm/wlmaker: unbreak build on non-x86 architectures

Upstream defines _POSIX_C_SOURCE to 1, 199309L, 200112L, 200809L in
multiple files, all of which hide SIGTRAP on FreeBSD. Drop POSIX as
BSD (unlike GNU) enable extensions by default.

submodules/libbase/subprocess.c:496:9: error: use of undeclared identifier 'SIGTRAP'

496 |         BS_ASSERT(rv == subprocess_ptr->pid);
    |         ^

submodules/libbase/assert.h:45:13: note: expanded from macro 'BS_ASSERT'

45 |             BS_ABORT();                                                 \
   |             ^

submodules/libbase/assert.h:34:28: note: expanded from macro 'BS_ABORT'

34 | #define BS_ABORT() { raise(SIGTRAP); }
   |                            ^

PR: 276743
Reported by: pkg-fallout (aarch64)