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.