BUG: Improve SWIG version check

See ITK-3303
The swig version was checked before SWIG_VERSION was defined when using system swig.
There is now also a new variable, ITK_SWIG_VERSION, which defines ITK's swig version,
and is propagated to SWIG_VERSION when using ITK's swig.

Change-Id: Idc755567691d32543997797a7d85443a1488a3ed
Merge topic 'SwigVersion'

55807b67 BUG: Improve SWIG version check