graphics/libraw: link against libomp with OPENMP=on
Since 07fb2d5e9d00 consumers - most notably shotwell - crashed hard
on RAW file operations when libraw was built with OPENMP=on:
ld-elf.so.1: /usr/local/lib/libraw.so.20: Undefined symbol "__kmpc_fork_call"
adding -lomp to libraw's LDFLAGS (via OPENMP_LDFLAGS, so it stays
conditional) fixes the issue.
Approved by: maintainer-timeout (jhale@)