graphics/darktable: LLVM 3.7 does not provide OpenMP on i386

This fixes the build on FreeBSD 10.x and 11-CURRENT on i386.

While here, completely remove the OPENMP option. The implementation was
incomplete anyway because disabling it would not turn off OpenMP in
darktable if it was built with GCC.

Reviewed by: kwm
Approved by: kwm
Differential Revision: https://reviews.freebsd.org/D4130