Some users where "clever" enough to supply a library in LDFLAGS/LIBS
thath contained omp_get_num_threads, therefore tricking configure into
thinking that we do not need any flags to enable OpenMP.

Now the Fortran test only works if "use omp_lib" and "!$" OpenMP
conditional compilation work.

Also, if no valid OpenMP flag could be detected configure silently
continued. I changed this to an explicit error.