Improve OpenMP detection in configure.ac
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.
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