Fix build with Clang for libpng versions 1.4.0 to 1.5.2

Fix build with Clang for libpng versions 1.4.0 to 1.5.2

Versions 1.4.0 to 1.5.2 of libpng declare png_longjmp_ptr to
have a noreturn attribute if PNG_PEDANTIC_WARNINGS_SUPPORTED
is enabled, but most declarations of longjmp in the wild do
not add this attribute. This causes problems when the png_jmpbuf
macro expands to calling png_set_longjmp_fn with a mismatched
longjmp, as compilers such as Clang will treat this as an error.

To work around this we override the png_jmpbuf macro to cast
longjmp to a png_longjmp_ptr.

See also http://llvm.org/bugs/show_bug.cgi?id=10338

Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>
Change-Id: I197cfa12af76410310e409bc0fce7d4332ee66a6
Reviewed-on: http://codereview.qt.nokia.com/3929
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>