Incorporating changes from Slicer and improve style

Adding changes in Slicer's version of the file which have a include
guard and a sanity check for Swig_DIR.

Additionally renamed version variable to be prefixed with SWIG.

Added variable with define the SWIG version, and the hash in one place
to aid in all getting updated when a version changes.

Change-Id: I5bc26b2ec425f6e21e9822df08d28342352f6252
Moving dependency graph to end, to print all

Adding missing DEPENDENCIES variable to SimpleITK examples external
project.

Using Swig DEPENDENCIES variable in ExternalProject call.

Change-Id: Ic1a9913034354c48c80a34b4e7caad8c73018505
Extracting PCRE external project into separate file

Change-Id: Ib5cf08286bfc056709b004164261a0a7dfd4edbd
Remove unused FFT superbuild logic branch

Change-Id: I9a39b54d3b72d5a47fcf074e4a4d2e84a20820f8
Changing download URLS from SourceForge to Midas.

Change-Id: Ie5f26335e62591c0f9726bea32fa41f2507dcbeb
Merge topic 'SIMPLEITK-351_RefactorExternalSWIG' into next

8708bbb Changing download URLS from SourceForge to Midas.
2c6fb74 Remove unused FFT superbuild logic branch
538bcd1 Extracting PCRE external project into separate file
959f585 Moving dependency graph to end, to print all
95e092c Incorporating changes from Slicer and improve style