Qt5 doesn't build on Mac OS X v10.6.7

Qt5 is failing to build on SnowLeopard. The build error happens when it meets the file qtbase/src/plugins/platforms/cocoa/qnsview.mm:
qnsview.mm: In function 'void -[QNSView handleMouseEvent:](QNSView*, _message_ref_t*, NSEvent*)':
qnsview.mm:172: warning: 'NSWindow' may not respond to '-convertRectToScreen:'
qnsview.mm:172: warning: (Messages without a matching method signature
qnsview.mm:172: warning: will be assumed to return 'id' and accept
qnsview.mm:172: warning: '...' as arguments.)
qnsview.mm:172: error: conversion from 'objc_object*' to non-scalar type 'CGRect' requested
make[5]: *** [.obj/debug-shared/qnsview.o] Error 1

The problem is that convertRectToScreen is only available since v10.7, as it says on the documentation here: http://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSWindow_Class/Reference/Reference.html#//apple_ref/occ/instm/NSWindow/convertRectToScreen: