Use CMAKE_SYSROOT to manage the sysroot automatically when cross-compiling instead of adding the '--sysroot' flag manually everywhere. We only need to add it manually in places where CMake does not, like in our custom GCC/Clang response file for PCH.
Differentiate between setting up header search path for system headers and for application headers so that GCC/Clang may suppress compilation warnings from the former, if any.
When cross-compiling we have already defined '--sysroot' to find the direct library dependencies. However when a shared library has its own dependencies (DT_NEEDED) then we need to supply extra linker flags to help the linker to find those indirect library dependecies in the sysroot.
Actually we also had this problem for 32-bit ARM cross-compiling, but we cheated in the earlier CI build jobs by using Linaro/Raspbian compiler toolchain. Somehow the Raspbian's linker simply "ignores" those indirect library dependencies or it uses other mean to locate them (but I doubt that), and so it does not have linker issues with libreadline.so (depends on tinfo), libGLESv2.so (depends on glapi), etc. The other ARM compiler toolchains from Linaro seem to be more strict in this regard. Most likely the raspbian's linker is a gold linker.
This commit could also potentially fix a known issue in our FindUrho3D.cmake module where we have to hardcode the library dependencies that libUrho3D.so in turn depends on (to be investigated later).
