When building under 5.1 on Mac OS X 10.8, the -isysroot flag is passed to the compiler, which prevents it from seeing any libraries/headers not present in the -isysroot path. For example, if I have the Evernote SDK framework (available from https://github.com/evernote/evernote-sdk-mac) installed under /Library/Frameworks, 5.1 can not see it to use its headers or link against it. 5.0.x can.

This problem can be worked around by linking the necessary frameworks into the SDK root given in the -isysroot flag. Adding additional search paths to the .pro file does not appear to work (unless I am doing it wrong), presumably because they also are "rooted" in the -isysroot path.

Example failed command under 5.1:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -c -pipe -O2 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk -mmacosx-version-min=10.6 -Wall -W -DQT_NO_DEBUG -DQT_PRINTSUPPORT_LIB -DQT_WIDGETS_LIB -DQT_SQL_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I/usr/local/qt-5.1.0b/mkspecs/macx-clang -I../EZPReport -I/usr/local/qt-5.1.0b/include -I/usr/local/qt-5.1.0b/include/QtPrintSupport -I/usr/local/qt-5.1.0b/lib/QtPrintSupport.framework/Versions/5/Headers -I/usr/local/qt-5.1.0b/include/QtWidgets -I/usr/local/qt-5.1.0b/lib/QtWidgets.framework/Versions/5/Headers -I/usr/local/qt-5.1.0b/include/QtSql -I/usr/local/qt-5.1.0b/lib/QtSql.framework/Versions/5/Headers -I/usr/local/qt-5.1.0b/include/QtGui -I/usr/local/qt-5.1.0b/lib/QtGui.framework/Versions/5/Headers -I/usr/local/qt-5.1.0b/include/QtCore -I/usr/local/qt-5.1.0b/lib/QtCore.framework/Versions/5/Headers -I. -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk/System/Library/Frameworks/OpenGL.framework/Versions/A/Headers -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk/System/Library/Frameworks/AGL.framework/Headers -I. -I. ../EZPReport/EvernoteInterface.mm -o EvernoteInterface.o
../EZPReport/EvernoteInterface.mm:20:9: fatal error: 'EvernoteSDK-Mac/EvernoteSDK.h' file not found
#import <EvernoteSDK-Mac/EvernoteSDK.h>
^
1 error generated.
make: *** [EvernoteInterface.o] Error 1
12:36:24: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project EZPReport (kit: Desktop-Qt-5.1.0b)
When executing step 'Make'

Same file building properly under 5.0.1:

clang -c -pipe -mmacosx-version-min=10.6 -O2 -Wall -W -DQT_NO_DEBUG -DQT_PRINTSUPPORT_LIB -DQT_WIDGETS_LIB -DQT_SQL_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I/usr/local/qt-5.0.1/mkspecs/macx-clang -I../EZPReport -I/usr/local/qt-5.0.1/include -I/usr/local/qt-5.0.1/include/QtPrintSupport -I/usr/local/qt-5.0.1/lib/QtPrintSupport.framework/Versions/5/Headers -I/usr/local/qt-5.0.1/include/QtWidgets -I/usr/local/qt-5.0.1/lib/QtWidgets.framework/Versions/5/Headers -I/usr/local/qt-5.0.1/include/QtSql -I/usr/local/qt-5.0.1/lib/QtSql.framework/Versions/5/Headers -I/usr/local/qt-5.0.1/include/QtGui -I/usr/local/qt-5.0.1/lib/QtGui.framework/Versions/5/Headers -I/usr/local/qt-5.0.1/include/QtCore -I/usr/local/qt-5.0.1/lib/QtCore.framework/Versions/5/Headers -I. -I/System/Library/Frameworks/OpenGL.framework/Versions/A/Headers -I/System/Library/Frameworks/AGL.framework/Headers -I. -I. ../EZPReport/EvernoteInterface.mm -o EvernoteInterface.o