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 210b0b99a9
(XL: Fix using Fortran modules from their output directory, 2020-02-28,
v3.18.0-rc1~640^2~1).

Modules/Compiler/NAG-Fortran.cmake

@@ -31,6 +31,7 @@ endif()
 set(CMAKE_Fortran_SUBMODULE_SEP ".")
 set(CMAKE_Fortran_SUBMODULE_EXT ".sub")
 set(CMAKE_Fortran_MODDIR_FLAG "-mdir ")
+set(CMAKE_Fortran_MODDIR_INCLUDE_FLAG "-I") # -mdir does not affect search path
 set(CMAKE_SHARED_LIBRARY_Fortran_FLAGS "-PIC")
 set(CMAKE_Fortran_FORMAT_FIXED_FLAG "-fixed")
 set(CMAKE_Fortran_FORMAT_FREE_FLAG "-free")