Move configuration of global headers

Previously it was possible to get a different result from cmake .. && make && make install than one got from cmake .. && cmake .. && make && make install because the lmfit detection ran after config.h was generated, so HAVE_LMFIT changed value from one cmake to the next, triggering a rebuild of all source files that depended on config.h. lmfit is supposed to fall back on the internal version, but did not, so this is buggy. It's also very inefficient with compilation in some workflows.