Create the m4 dir if it doesn't exist.

aclocal version 1.13 fails with the following error:

aclocal: error: couldn't open directory 'm4': No such file or directory

Creating the m4 directory before executing aclocal fixes the error.
Merge pull request #1 from benburkert/patch-1

Create the m4 dir if it doesn't exist.
Merge branch 'master' of https://github.com/max-horvath/htop-osx