package/eigen: override Fortran path

package/eigen: override Fortran path

The language detection is falling back to the host system
Fortran compiler. An example of this is in RHEL7.9
(gcc4.8.5 20150623 (Red Hat 4.8.5-44)).

This patch bypasses detection and points to the location
where the compiler would be installed (if present). In the
cases where it doesn't exist, the detection falls through
and leaves Fortran disabled.

Fixes:
http://autobuild.buildroot.net/results/8354da225d1e5e337aa7ea62a7e6524fb5f1135f/

Signed-off-by: Matthew Weber <matthew.weber@collins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>