CMake autotests fail when cross-compiling

The compile time tests for CMake fail when cross-compiling for embedded.
There are linker errors shown because the linker is unable to find system libraries.
It looks like important compiler options are missing from the calls like SYSROOT, CFLAGS and CXXFLAGS.

I have disabled the compile time tests for embedded for now (https://codereview.qt-project.org/125531).

I already tried to set CMAKE_SYSROOT, CMAKE_C_FLAGS and CMAKE_CXX_FLAGS. But these seem only to have effect on the initial checks but are not passed further to the actual test execution.

Solution would be to somehow pass these parameter from the Qt mkspec to CMake.