Fix MS-Windows build as followup to pdumper executable lookup

Fix MS-Windows build as followup to pdumper executable lookup

* src/w32.c (w32_my_exename): New function.
* src/w32.h (w32_my_exename): Add prototype.
* src/emacs.c (load_pdump_find_executable) [WINDOWSNT]: Find
the actual file name of the program without looking along
PATH, by calling w32_my_exename.

* nt/mingw-cfg.site (ac_cv_func_canonicalize_file_name)
(ac_cv_func_realpath, gl_cv_func_realpath_works): Disable
testing.
* nt/gnulib-cfg.mk (OMIT_GNULIB_MODULE_canonicalize-lgpl): Set
to true.