The conversion of Check<Lang>CompilerFlag, SourceCompiles, and
SourceRuns over to the new functions has the possibility of breaking
projects that had functions with those existing names.
To reduce the possibility of collisions we now have all the
legacy code call functions that start with `cmake_`, and users
will need to explicitly include the new modules to get the
non-prefixed versions
Fixes: #21359
In commit 357e2ef429 (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 10ae907de0 (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
