Apple: Enable -isystem for GNU Compiler >= 4 (#15953)
Due to #4662 -isystem support was disabled for all GNU Compilers on Apple platforms. But the change was probably a just work around for a broken compiler on Tiger (see 10837#c27206). So we tighten the condition to only kick in for GCC versions earlier than 4. That should ensure sane behavior for Xcode 3.2 and later.