STYLE: Set ITK_USE_SYTEM_ZLIB and ITK_USE_SYTEM_SZIP

instead of ZLIB_USE_EXTERNAL and SZIP_USE_EXTERNAL
The flags were set to ON and OFF instead of 1 and 0 and the description was
extended a little bit.

See ITK-3157

Change-Id: I83f1e1bc68a89d2d2b708da535125b14033141bf
Merge topic 'UseSystem'

f51b3ed1 STYLE: Set ITK_USE_SYTEM_ZLIB and ITK_USE_SYTEM_SZIP