Hubertus van Dam authored Aug 28, 2018 and Brad King committed Aug 30, 2018

Run the Check_Fortran_Libraries macro with an *empty* list of libraries
to detect whether the compiler implicitly links BLAS.  If this works,
set `BLAS_LIBRARIES` to a placeholder value to get through the rest of
our logic.  At the end replace the placeholder by a real empty string
again to report to callers.