Dont force ../.deps in third-party/CMakeLists.txt

- third-party is built under .deps by default instead of using its own
  ${CMAKE_BINARY_DIR}, move this default setting out of the cmake
  settings and into the Makefile.
- As a consequence the workflow of building third-party using CMake
  should feel more natural, avoid the additional folder or setting
  DEPS_DIR from the command line.
- This commit does not change the default behaviour when calling the
  Makefile wrapper.
Allow cmake caller to override DEPS_PREFIX

- Caller can override bundled dependency location using
  DEPS_PREFIX
- Cache variable DEPS_PREFIX, using .deps/usr by default
- Removed unused variables DEPS_BIN_DIR, DEPS_BUILD_DIR, DEPS_DIR
  DEPS_INSTALL_DIR
- Corner case: if the caller tries to override DEPS_PREFIX after a
  successful cmake configuration, the caller needs to clear the cache
  because dependency checks are based on the old value
Merge pull request #1588 from equalsraf/tb-no-central-deps

Dont force ../.deps in third-party/CMakeLists.txt use the Makefile instead