Fix openmp compile error

[jakemclaughlin6]

Fixes the following error when compiling:

/usr/bin/ld: /usr/local/lib/libceres.a(local_parameterization.cc.o): undefined reference to symbol 'omp_get_max_threads@@OMP_1.0'
/usr/bin/ld: /lib/x86_64-linux-gnu/libgomp.so.1: error adding symbols: DSO missing from command line

[svwilliams]

Do you know if this is still an issue? Nothing in fuse_optimizers uses OpenMP directly. So the issue must be some missing dependency somewhere. I know it's been forever since this patch was submitted.