[cmake] Reset variable before using it

A bunch of macros use the same variable name, and since CMake macros
don't get their own scope, the value persists across macro invocations,
and we can end up exporting targets which shouldn't be exported. Clear
the variable before each use to avoid this.

Converting these macros to functions would also help, since it would
avoid the variable leaking into its parent scope, and that's something I
plan to follow up with. It won't fully address the problem, however,
since functions still inherit variables from their parent scopes, so if
someone in the parent scope just happened to use the same variable name
we'd still have the same issue.

llvm-svn: 357036