On Windows: enable detecting memcache support from apr-util 1.3 or later.
* build/generator/gen_win_dependencies.py (find_libraries): Update caller. (_find_apr_util_and_expat): Rename to... (_find_apr_util_etc): ... this. Declare availability of memcache support when we have apr-util 1.3 or later.
Add gen-make generator support for Visual Studio "2015"
* build/generator/gen_win_dependencies.py Document support for VS 2015.
* gen-make.py Add magic values for VS2015. Stop guessing that we have VS 7.0 on future versions, as VS 2012 is a much better starting point for future VS versions (as that has at least the latest .vcxproj file support, which 7.0 didn't have.)