-Wno-error=unused-but-set-variable requires GCC >= 4.6

In qtwebkit/Source/ThirdParty/leveldb/Target.pri

gcc {
QMAKE_CXXFLAGS_WARN_ON += -Wno-error=unused-but-set-variable
QMAKE_CXXFLAGS += -Wno-error=unused-but-set-variable
}

-Wno-error=unused-but-set-variable was introduced in gcc 4.6.
Trying to use in older versions causes a compile error:

cc1plus: error: -Werror=unused-but-set-variable: No option -Wunused-but-set-variable

This is in webkit, but the Target.pri file appears to be part of the Qt build system used to build webkit for qtwebkit.

Not sure if you want to support gcc pre 4.6, but I wanted to report so it is known.