osg3: Fix building on OSX 10.8

We can't use our own compilers. We must use Apple's for their block
extension, which their headers for Cocoa use heavily. Such is life.

Packages.py

@@ -806,10 +806,21 @@ class osg3(CMakePackage):
     patches = 'patches/osg3'
 
     def configure(self):
+        other_flags = ['-DBUILD_OSG_APPLICATIONS=ON', '-DCMAKE_VERBOSE_MAKEFILE=ON', '-DOSG_USE_QT=OFF', '-DBUILD_DOCUMENTATION=OFF']
+        if self.arch.os == 'osx':
+            other_flags.extend(['-DOSG_DEFAULT_IMAGE_PLUGIN_FOR_OSX=imageio','-DOSG_WINDOWING_SYSTEM=Cocoa'])
+            # Cocoa bindings can't be built unless using an apple
+            # provided compiler. Using a homebrew or macports built
+            # GCC or hand built clang will be missing the required
+            # 'blocks' extension. The error will look something like
+            # "NSTask.h: error: expected unqualified-id before '^'
+            # token".
+            self.env['CXX'] = 'g++'
+            self.env['CC'] = 'gcc'
         super(osg3, self).configure(
             with_='GDAL GLUT JPEG OpenEXR PNG ZLIB CURL QuickTime CoreVideo QTKit'.split(),
             without='COLLADA FBX FFmpeg FLTK FOX FreeType GIFLIB Inventor ITK Jasper LibVNCServer OpenAL OpenVRML OurDCMTK Performer Qt3 Qt4 SDL TIFF wxWidgets Xine XUL RSVG NVTT DirectInput GtkGL Poppler-glib GTA'.split(),
-            other=['-DBUILD_OSG_APPLICATIONS=ON', '-DCMAKE_VERBOSE_MAKEFILE=ON', '-DOSG_USE_QT=OFF'])
+            other=other_flags)
 
 class flann(CMakePackage):
     src = 'http://people.cs.ubc.ca/~mariusm/uploads/FLANN/flann-1.8.4-src.zip'