out-of-source API documentation build (1/2): rename doxygen.conf to doxygen.conf.tmpl
out-of-source API documentation build (2/2): update CMakeLists.txt and doxygen.conf.tmpl

docs/doxygen.conf.tmp: take input files from @SOURCE_DIR@/
docs/MakeLists.txt:
  - create ${CMAKE_CURRENT_BINARY_DIR}/docs/doxygen.conf from template
  - generate all docs targets in ${CMAKE_CURRENT_BINARY_DIR}
  - install all docs targets to ${CMAKE_INSTALL_PREFIX}
API docs: stop generating latex