GNU: Do not use -isystem with gfortran

The compiler documents that USE statements search for ".mod" files
in directories specified with -I, but not -isystem.

Reported-by: Hugh Sorby <[email protected]>

Modules/Compiler/GNU-Fortran.cmake

@@ -8,5 +8,8 @@ set(CMAKE_Fortran_FORMAT_FREE_FLAG "-ffree-form")
 set(CMAKE_Fortran_FLAGS_MINSIZEREL_INIT "-Os")
 set(CMAKE_Fortran_FLAGS_RELEASE_INIT "-O3")
 
+# No -isystem for Fortran because it will not find .mod files.
+unset(CMAKE_INCLUDE_SYSTEM_FLAG_Fortran)
+
 # Fortran-specific feature flags.
 set(CMAKE_Fortran_MODDIR_FLAG -J)