Fix #148; More robust feature detection


PR #125 added the string Magick::Magick_features

Issue #148 reported compilation failures on
```plain
  Mac OSX Yosemite
  Rails 4.2.0.beta4
  rake, version 10.4.0
  ImageMagick 6.8.9-8 Q16 x86_64 (via macports)

in rmmain.c:1724:28:
error: use of undeclared identifier 'MagickSupport'

This PR now makes the flow to use if defined:
GetMagickFeatures() or
  MagickFeatures or
  MagickSupport or
  "unknown"
```
I think this might be due to whether the `extconf.h` is generated with this symbol before the pre-processor check for it.  Otherwise, I'm not sure.