[CMake] Do not build targets whose dependencies are not available

[Chris Stankevitz]

Hello,

I have a library target ("a") that is a dependency of an executable
("b").  I have many more libraries and many more executable, some with
"complex" dependencies on the others.  Sometimes target "a" cannot be
built either because the user does not want it built or because
FIND_PACKAGE failed to find a dependency of "a".

Q: Can cmake automatically not build "b" when "a" is not available?

I hope the answer is "yes" otherwise I will have to add IF(HAVE_A)
code throughout my CMakeLists.txt file.  Worse, in my complex file I
will have to add lots of IF(HAVE_C), IF(HAVE_D) etc.

Thank you,

Chris