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