Fix legacy builds crashing in 10.9-10.11 by disabling clock_gettime

Recent Vim builds introduced the use of clock_gettime(), which on macOS
was only introduced in 10.12. In Vim, we added a configure check to
detect its existence (https://github.com/vim/vim/issues/12242) but it
only works to detect the build-time environment. For MacVim builds we
build on new machines but deploy on old machines by using
MACOSX_DEPLOYMENT_TARGET=10.9 when building. This means that the
configure script will detect clock_gettime() exists and includes it, but
at runtime the app will crash when running MacVim (legacy builds) in
10.9-10.11.

To fix this, add explicit checks to make sure we undef
HAVE_CLOCK_GETTIME if deployment target is below 10.12. Unfortunately
there isn't really a way to detect such things automatically in a
configure script as it requires knowledge of historical releases.

.github/workflows/ci-macvim.yaml

@@ -237,6 +237,7 @@ jobs:
       - name: Check version
         run: |
           ${VIM_BIN} --version
+          ${VIM_BIN} -u NONE -i NONE --not-a-term -esNX -V1 -c 'echo "\nprof_nsec:" .. has("prof_nsec") .. "\n"' -c quit
           ${VIM_BIN} -u NONE -i NONE --not-a-term -esNX -V1 -S ci/if_ver-1.vim -c quit
           ${VIM_BIN} -u NONE -i NONE --not-a-term -esNX -V1 -S ci/if_ver-2.vim -c quit