When cross compiling (using ROOT=), the build time dependencies are installed on the host (with the host's CFLAGS), and if the build time dependency is a static library, it's also copied from the host. Setting SYSROOT, --sysroot, --root-deps does not make a difference. They just install the dependency in the ROOT but do not use it.
That really depends on the build system of the package in question, not portage.
Also, your cross-compiler (gcc, clang, etc) should be configured to look for libraries in SYSROOT instead of /. A tool like crossdev will set that up for you.
So, this will not be possible with plane ROOT?
Is there any documentation on how it can be achieved manually?
Looks like I had hit this issue twice. *** This bug has been marked as a duplicate of bug 659766 ***