Disable webp also for ios

Already disabled for android and winrt; seems that libwebp's use of
neon does not compile.

Change-Id: Ia119244da1ae3c92f69dfe5ed91d20e665102958
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Avoid implicit function declaration compile errors in libjasper

Compilation for macx-ios-clang_OSX_10.9 would fail since the
jas_eprintf function was used undeclared. Fix by #including
jas_debug.h before4 use of that function.

Also restructure some code in jpc_tsfb.c to prevent the same
issue for jpc_tsfb_synthesize2 and jpc_tsfb_analyze2.

Change-Id: Ib5b444873af6aa756d1c77145acdd863eaca0bd2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Merge remote-tracking branch 'origin/5.4.0' into 5.4

Change-Id: I9002ef6585513ebfb9e641e890f8b0e855c67a08
Don't try to use incompatible system libwebp

Make the config test check compatibility more thoroughly.

Task-number: QTBUG-42850
Change-Id: I0771ee633868ad9a7ef03eee2434504d40bc9ce4
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>
Merge remote-tracking branch 'origin/5.4' into dev

Change-Id: I8b76d2098cc2d8bca5a63798d53182144d6a4442