draco: Improve installation and packaging support.

- Fixed omission of draco_version.h when installed in static
  configurations.
- Remove DracoConfig.cmake: it was broken and we actually use
  draco-config.cmake.
- Update FindDraco.cmake so it actually has a chance of working
  properly. Fixed usage of find_path() and subsequent related errors.
- Correct the usage of CMakePackageConfigHelpers in installation target setup
  and the config template.
- Add a CMake version file.
- Normalize the Draco variables exposed in CMake.
  - draco_FOUND -> DRACO_FOUND
  - DRACO_INCLUDE_DIRS, draco_INCLUDE_DIRS -> DRACO_INCLUDE_DIR
  - DRACO_LIBRARIES, draco_LIBRARIES -> DRACO_LIBRARY
  - DRACO_LIBRARY_DIRS, draco_LIBRARY_DIRS -> DRACO_LIBRARY_DIR
  - draco_VERSION_STRING -> DRACO_VERSION
- Use full path variants of GNUInstallDirs variables to init our
  install paths.

Fixes google#764

* Add install() for draco-version.cmake.

CMakeLists.txt

@@ -35,7 +35,6 @@ if("${draco_root}" STREQUAL "${draco_build}")
                 "And re-run CMake from the draco_build directory.")
 endif()
 
-include(CMakePackageConfigHelpers)
 include(FindPythonInterp)
 include("${draco_root}/cmake/draco_build_definitions.cmake")
 include("${draco_root}/cmake/draco_cpu_detection.cmake")
@@ -347,6 +346,7 @@ list(APPEND draco_core_sources
             "${draco_src_root}/core/draco_index_type_vector.h"
             "${draco_src_root}/core/draco_types.cc"
             "${draco_src_root}/core/draco_types.h"
+            "${draco_src_root}/core/draco_version.h"
             "${draco_src_root}/core/encoder_buffer.cc"
             "${draco_src_root}/core/encoder_buffer.h"
             "${draco_src_root}/core/hash_utils.cc"