Process all arguments in a single loop using a simple state machine.
While at it, fix some error message typos.  Also allow LINK_LIBRARIES
with no actual libraries to disable use of the -DLINK_LIBRARIES=...
from the CMAKE_FLAGS.  This was already possible in the old logic if
LINK_LIBRARIES was immediately followed by another keyword argument
instead of the end of the argument list, so allow it in general.
Update the RunCMake.try_compile test cases accordingly.