NAG: Fix using Fortran modules from their output directory
The NAG Fortran compiler's `-mdir` flag sets the module output directory but does not add the directory to the search path for using modules. This is inconsistent with other compilers like the GNU Fortran compiler's `-J` flag that does both. In order to make these consistent, add the module output directory with a `-I` flag on the NAG Fortran compiler so that it will be searched when using modules too. We already do this for the XL Fortran compiler since commit 210b0b99 (XL: Fix using Fortran modules from their output directory, 2020-02-28, v3.18.0-rc1~640^2~1).