CMakePackageConfigHelpers: Remove unused variable
CMakePackageConfigHelpers: restructure documentation and document commands
CMakePackageConfigHelpers: Add BUILD_TREE option

This option instructs configure_package_config_file to consider paths
that are not absolute as relative to the CMAKE_BINARY_DIR directory
instead of relative to the CMAKE_INSTALL_PREFIX` directory.

The new signature is therefore:

  configure_package_config_file(<input> <output> INSTALL_DESTINATION <path>
                                                 [PATH_VARS <var1> <var2> ... <varN>]
                                                 [NO_SET_AND_CHECK_MACRO]
                                                 [NO_CHECK_REQUIRED_COMPONENTS_MACRO]
                                                 [BUILD_TREE])
CMakePackageConfigHelpers: Add unit tests for BUILD_TREE option
Merge topic 'CMakePackageConfigHelpers_build_tree' into next

8bab4454 CMakePackageConfigHelpers: Add unit tests for BUILD_TREE option
b01fdb58 CMakePackageConfigHelpers: Add BUILD_TREE option
9528e6a6 CMakePackageConfigHelpers: restructure documentation and document commands
855e76e9 CMakePackageConfigHelpers: Remove unused variable