Build QTWebEngine from Qt out of devel rpms for centos 6.6

[Klaus47]

Hallo,

I have a Little Problem to build qtwebengine 5.5.1 from qtwebengine-opensource-src-5.5.1.tar.gz on Centos 6.6.
I have installed the binary and devel Packages of QT 5.5.1 from RPMs of a given repository.
Then I install the red hat devtoolset-2 to get the G++ 4.8 Compiler which is necessary to build the qtwebengine.
Than I start qmake and I get the following Output:

Using gcc Version 4.4, but at least gcc Version 4.7 is required to build Qt WebEngine.
QtWebEngine will not be built.

What can I do? I think the binary and devel rpms are build with gcc/g++ 4.4 and this is the Problem.
How can I solve the Problem?

Sincerely

Klaus