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"
```
per https://github.com/gemhome/rmagick/commit/b9a26ca9fab9f2844b22442b6b22d741dc301623#commitcomment-8827966 h/t @u338steven

```MagickSupport``` exists from ImageMagick ver.6.4.8-10 to 6.5.7

ver.6.4.8-10:
http://sourceforge.net/projects/imagemagick/files/old-sources/6.x/6.4/

version.h:
```c
#define MagickSupport MagickHDRISupport MagickOPENMPSupport
```


```GetMagickFeatures``` exists from 6.5.7-1 onwards

ver.6.5.7-1:
http://trac.imagemagick.org/changeset/404/ImageMagick/trunk/magick/version.h

Removed ```MagickSupport```, added ```GetMagickFeatures```
 ---

trunk(ver.7.0.0):
http://trac.imagemagick.org/browser/ImageMagick/trunk/MagickCore/version.h

```GetMagickFeatures``` exists