QT Creator/qmake ignores a custom compiler path specified for a chosen kit. Instead, it generates a makefile with the Xcode versions of GCC and G++ for the CC and CXX rather than the paths chosen in the kit.

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The makefiles generated by the qmake of Qt5.0.2 and Qt5.1.0 have some difference
It will always select the default compiler come with XCode(if I interpret the makefile correctly)

The make file generated by qmake come with Qt5.1.0 beta

// Compiler, tools and options

CC = /Applications/Xcode.app/Contents/Developer/usr/bin/gcc
CXX = /Applications/Xcode.app/Contents/Developer/usr/bin/g++
.......
LINK = /Applications/Xcode.app/Contents/Developer/usr/bin/g++

// Output directory

The makefile generated by Qt5.0.2

// Compiler, tools and options

CC = gcc
CXX = g++
.......
LINK = g++

// Output directory

There are some problems if the compiler switch to clang too
The compiler will always be the default compiler come with XCode