CMake: cosmetic change in error message

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
CMake: fix slash in variable

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
CMake: don't use changed project-root path to lookup 'sh'

this bites in a cross-compile environment, where no executable 'sh' is
present in the given sysroot.

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>