mapserver: add installation of headers

Matches layout used and files installed by:
- libmapserver-6.2.1-dev in Ubuntu saucy: [libmapserver-6.2.1-dev](http://packages.ubuntu.com/saucy/amd64/libmapserver-6.2.1-dev/filelist)
- UbuntuGIS (most current open source GIS repo):  [ubuntugis-unstable](https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable/+packages?batch=75&memo=75&start=75), [libmapserver-6.2.1-dev_6.2.1-6~raring1_amd64.deb](https://launchpadlibrarian.net/148201891/libmapserver-6.2.1-dev_6.2.1-6%7Eraring1_amd64.deb)

Addresses https://github.com/mxcl/homebrew/issues/24949

IMO, the installation of the headers should be by default, i.e. no need for a formula option.