mapserver: update to 6.4 and new cmake build

Update to 6.4 and new CMake build. This is a **work-in-progress**; any opinions appreciated. I don't personally use all of these features, e.g. Java/Ruby/PHP modules, MySQL, etc., and would like to see those users that do test the formula.

Options set relative to project's [INSTALL.CMAKE](https://github.com/mapserver/mapserver/blob/branch-6-4/INSTALL.CMAKE#L122-L158) doc. The following options were switched ON, because they did not incur any further dependencies:
 * `WITH_PYTHON` (as per previous formula)
 * `WITH_KML`
 * `WITH_CURL`, `WITH_CLIENT_WMS`, `WITH_CLIENT_WFS`

I setup building and installing of all common mapscript languages to `prefix/'mapscript/<lang>'`, since they generally require `sudo` for installation to the base system locations. No support for brewed PHP, Ruby or Perl for the modules at this time.

For SVG symbology support I used `WITH_RSVG` instead of `WITH_SVGCAIRO`, because the latter had the following run-time loading error (here with Python mapscript):
```
>>> import mapscript
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/mapscript.py", line 28, in <module>
    _mapscript = swig_import_helper()
  File "/usr/local/lib/python2.7/site-packages/mapscript.py", line 24, in swig_import_helper
    _mod = imp.load_module('_mapscript', fp, pathname, description)
ImportError: dlopen(/usr/local/lib/python2.7/site-packages/_mapscript.so, 2): \
Symbol not found: _png_set_gray_1_2_4_to_8
  Referenced from: /usr/local/lib/libsvg.1.0.0.dylib
  Expected in: flat namespace
 in /usr/local/lib/libsvg.1.0.0.dylib
```
Added HEAD build and optional generation of local docs.