gcc-4.5.1: make c++ include path relative to "--sysroot"
So far c++ include path is not relative to "--sysroot", which brings
trouble if we want to use the toolchain in a new environment where
the original build directory generating that toolchain is not
available. It's firstly exposed in multiple SDK sysroots support, and
then in the case when sstate packages are used, where c++ standard
headers are missing because gcc tries to search original build dir.
This patch makes c++ include path now relative to "--sysroot", and
then once "--sysroot" is assigned correctly in new environment, c++
include paths can be searched as expected.
