WebPConfig.cmake.in: use calculated include path

when setting WebP_INCLUDE_DIRS; this value is relative to the
WebPConfig.cmake install location (${PACKAGE_PREFIX_DIR}/include) and
allows the install to be relocated after creation.

See also:
https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html#example-generating-package-files

Bug: webp:610
Change-Id: If877625e96584a9808b0bba902e3f0b2a744aa60
Fixed: webp:610

CMakeLists.txt

@@ -743,7 +743,8 @@ endif()
 configure_package_config_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/cmake/WebPConfig.cmake.in
   ${CMAKE_CURRENT_BINARY_DIR}/WebPConfig.cmake
-  INSTALL_DESTINATION ${ConfigPackageLocation})
+  INSTALL_DESTINATION ${ConfigPackageLocation}
+  PATH_VARS CMAKE_INSTALL_INCLUDEDIR)
 
 # Install the generated CMake files.
 install(FILES "${CMAKE_CURRENT_BINARY_DIR}/WebPConfigVersion.cmake"

cmake/WebPConfig.cmake.in

@@ -10,7 +10,9 @@ endif()
 
 include ("${CMAKE_CURRENT_LIST_DIR}/@[email protected]")
 
-set(WebP_INCLUDE_DIRS "@CMAKE_INSTALL_FULL_INCLUDEDIR@")
+set_and_check(WebP_INCLUDE_DIR "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
 set(WEBP_INCLUDE_DIRS ${WebP_INCLUDE_DIRS})
 set(WebP_LIBRARIES "@INSTALLED_LIBRARIES@")
 set(WEBP_LIBRARIES "${WebP_LIBRARIES}")
+
+check_required_components(WebP)