#119 build script fix for iphone

Unstable_(example)
open
nobody
1
2012-10-11
2012-10-11
Aviad Rozenhek
No

poco/build/config/iPhone

IPHONE_SDK_ROOT ?= $(shell xcode-select -print-path)/Platforms/$(IPHONE_SDK).platform/Developer/SDKs

Just added the ? infront of the =

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