CMake: use Requires: for .pc generation (don't parse link flags)

Instead of trying to translate/parse the INTERFACE link flags from CMake
to the .pc file, make use of the more native-to-pkg-config "Requires"
and "Requires.private", so that we can refer to the whole dependent .pc
pacakge name.

This fixes static linking when the dependencies of static libraries are
also static, and allows for using pkg-config all the way down.

When sasl and zlib are found, Rfb public headers also include headers
from these libraries so they are the only ones listed in "Requires".
The others can be in "Requires.private".

libsystemd and libpng are exclusively libvncserver dependencies, while
sasl is exclusively a libvncclient dependency. The rest of the libraries
are common dependencies to both.

Signed-off-by: Johannes Kauffmann <johanneskauffmann@hotmail.com>