COMP: Add a check for Blocks in GPU module

See bug: ITK-3331

When compiling with ITK_USE_GPU=ON and macports gcc 4.9.2,
the compilation fails with errors like these:
/usr/include/dispatch/object.h:143:15: error: expected unqualified-id before '^' token
 typedef void (^dispatch_block_t)(void);

This patch checks for a working Blocks extension implentation.

More comments and explanations can be found in the itkCheckHasBlocks.cxx file.

Change-Id: I22a8d0fae62fbf33713f53879f0cda08fabb3d90
Merge topic 'GPUCheckBlocks'

d2c5c8a6 COMP: Add a check for Blocks in GPU module