Add support for libxml as a system library target on iOS

Add support for libxml as a system library target on iOS

Restructures the gyp conditions to make the system-lib and non-system-lib paths much clearer, and to ensure that if a new OS tries to use use_system_libxml it won't silently use the non-system path.

BUG=None
TEST=None


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=146976

[stuartmorgan, 2012-07-16 18:24:24]

This is basically the same reworking that we did for libsql, since the
conditionals get really ugly otherwise.

[rohitrao (ping after 24h), 2012-07-17 07:40:12]

lgtm

[commit-bot: I haz the power, 2012-07-17 09:27:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/stuartmorgan@chromium.org/10787018/3001

[commit-bot: I haz the power, 2012-07-17 10:41:15]

Change committed as 146976