Also, remove !Makefile as we are now out of xrdp tree.

That can cause unexpected behavior, especially with third party tools.
The ignored files will be lost if the source tree is re-imported to
another git repository, unless special care is taken.

Whitelist all non-generated makefiles.

To whitelist instfiles/pam.d/xrdp-sesman, add path to all generated
executables listed in the top-level .gitignore, sort .gitignore
alphabetically.

Add mkinstalldir, it's used by Automake on some systems.