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).