PCH: Do not enable GNU or Intel PCH settings for Fortran
The PCH settings are shared by C and CXX languages but do not make sense
for Fortran. In particular, CMAKE_PCH_EXTENSION
should not be set
because it can overwrite the value set for C/C++ languages, which may
have a different compiler vendor than the Fortran compiler.
Fixes: #20752 (closed)
Backport: release:HEAD^2
Edited by Brad King