make install fails for different install locations of `bash_completion`

Running on OS X, having installed bash_completion through Homebrew my completion directory is not /etc/bash_completion.d/.

Instead it is /usr/local/etc/bash_completion.d/, or more general: $(brew --prefix)/etc/bash_completion.d/

Running make install I get the following error:

... installing git-summary
... installing git-touch
... installing git-undo
... installing git-unlock
cp -f man/git-*.1 "/usr/local/share/man/man1"
mkdir: /etc/bash_completion.d: Permission denied
make: *** [install] Error 1

This is because /etc/ is owned by root, /etc/bash_completion.d/ does not exist (nor should it!) and it is trying to mkdir as my current user.

I suggest adding in a check that if the user has brew and if `$(brew --prefix)/etc/bash_completion.d/' exists that then that directory is used, rather than the current hardcoded default.

Would a pull request for this be welcome?

Start with the install target invoked by make install and reproduce the failure on OS X with Homebrew's bash_completion location. Trace how the completion directory is selected, then verify that installation completes without a permission error when the completion files are under $(brew --prefix)/etc/bash_completion.d/.