Cannot use "=" with cpp.includePaths for Cross Compiling Sysroot

Setting up a cross compiling kit using a sysroot with Qbs is a charm, except that "cpp.includePaths" does not work as expected.
If you write

cpp.includePaths: "/usr/include/somelibsheaderdir"

qbs calls the compiler with

-I/usr/include/somelibsheaderdir

If you write

cpp.includePaths: "=/usr/include/somelibsheaderdir"

qbs calls the compiler with

-I/home/username/your/project/source/directory/=/usr/include/somelibsheaderdir

So there is no way to tell the compiler to append the passed sysroot to the include path (maybe the same with the library paths but I did not tested it yet).

I would prefer something like "cpp.prependSysroot: true/false" wich automaticly prepends the "=" to the include paths.
Example:

cpp.prependSysroot: true
cpp.includePaths: "/usr/include/somelibsheaderdir"

leads to

-I=/usr/include/somelibsheaderdir

Additionally if the include paths have the "=" prefix they should be treated like absolut paths (independent of "cpp.prependSysroot").