Revision 1835134

Fix CMake build on Unix:

* CMakeLists.txt:
   - Default string options to empty string instead of boolean "OFF".
   - Expose APR/-Util libraries as interface requirements of the
     shared Serf library, since its users will have to use APR functions.

* build/APRCommon.cmake, build/FindAPR.cmake, build/FindAPRUtil.cmake:
   Fix argument and result quoting when invoking apr-1-config etc.
* build/SerfVersion.cmake: Look for serf.h header in the known source directory.