Pierre Ossman
cd90e28dab
Prevent incorrect cmake usage
It's a reoccurring issue that users try to build individual components
by pointing cmake at a specific subdirectory, e.g. 'cmake vncviewer'.
CMake, unfortunately, has insufficient protection against this so we'll
need to add a manual check.
This commit only adds it to the most likely places for misuse so we
don't have to pollute every CMakeLists.txt.