CMakeLists.txt: Align binary locations with Autotools

Target "xmlwf" even collided with the source folder of the
same name, previously:

$ cmake . && make
...
[ 82%] Linking C executable xmlwf
.../ld: cannot open output file xmlwf: Is a directory
collect2: error: ld returned 1 exit status
...
1 file changed
tree: 200d4c881ea6786bfdcfd79c641d481dccf61963
  1. .gitignore
  2. expat/
  3. htdocs/
  4. testdata/