If we fail to create the generator named by CMAKE_GENERATOR, exit
with an error message instead of crashing. While at it, fix the
wording of the error message when CMAKE_GENERATOR is not set.
Extend the RunCMake.CommandLine test with cases covering the
"cmake --build" option when the named directory does not provide
a CMakeCache.txt with a valid CMAKE_GENERATOR.
The logic added by commit ffc0b5e4 (Overwrite the symlink if it already
exists, 2007-02-15) does not recognize and remove existing broken links
before replacing them. Improve the logic to remove any existing
destination file or link (but not directory). On failure, report an
error message explaining why the existing path could not be removed or
the new one could not be created.
Add a RunCMake.CommandLine test to cover 'cmake -E' cases. Start with
test cases covering 'cmake -E create_symlink' behavior on UNIX platforms.
Brad King