Michael Hirsch authored Oct 15, 2020 and Brad King committed Oct 15, 2020

In commit 357e2ef4 (CheckSoureRuns: Add a unified way to check if a
source runs, 2020-09-14, v3.19.0-rc1~118^2), the default Fortran source
file extension was accidentally changed from `.F90` to `.F`.  Fix that.

In commit 10ae907d (CheckSoureCompiles: Add a unified way to check if
a source compiles, 2020-09-14, v3.19.0-rc1~118^2~1), the default Fortran
source extension was correctly preserved as `.F`, but `.F90` is a better
default both for consistency and modern usage.  Use that for direct
calls to the `check_source_compiles` macro.  Update our original
`check_fortran_source_compiles` implementation to use `.F` by default
as it did before.

Fixes: #21307