CMake: use generator expressions to find output files

Multi-configuration generators, like Visual Studio, use a different
output directory (Debug, Release, etc.) for each configuration. To find
the output files reliably, switch to using generator expressions instead
of hardcoding the file paths for PDBs, export files, etc.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1752332 13f79535-47bb-0310-9956-ffa450edef68