FindPkgConfig: restructure documentation and document commands and variables
FindPkgConfig: Extend PKG_CONFIG_PATH using CMake variables

Use CMAKE_PREFIX_PATH, CMAKE_FRAMEWORK_PATH, and CMAKE_APPBUNDLE_PATH
cache and environment variables to extend PKG_CONFIG_PATH before calling
pkg-config.

In each of the path in these variables it searches for lib/pkgconfig.
Then, depending on the system, it searches for
lib/${CMAKE_LIBRARY_ARCHITECTURE}/pkgconfig (debian) or for
lib64/pkgconfig (other 64 bit unixes). If any of these path is found,
it is appended to the PKG_CONFIG_PATH enviromnent variable.

Add two new arguments to the pkg_check_module and pkg_search_module
macro, NO_CMAKE_PATH and NO_CMAKE_ENVIRONMENT_PATH. The new signature
are therefore:

   pkg_check_modules(<PREFIX> [REQUIRED] [QUIET]
                     [NO_CMAKE_PATH] [NO_CMAKE_ENVIRONMENT_PATH]
                     <MODULE> [<MODULE>]*)
   pkg_search_module(<PREFIX> [REQUIRED] [QUIET]
                     [NO_CMAKE_PATH] [NO_CMAKE_ENVIRONMENT_PATH]
                     <MODULE> [<MODULE>]*)

By default, if CMAKE_MINIMUM_REQUIRED_VERSION is 3.1 or later (in
order to keep compatibility with the previous behavior), or if
PKG_CONFIG_USE_CMAKE_PREFIX_PATH is set, the CMAKE_PREFIX_PATH,
CMAKE_FRAMEWORK_PATH, and CMAKE_APPBUNDLE_PATH cache and environment
variables will be added to pkgconfig search path.

The NO_CMAKE_PATH and NO_CMAKE_ENVIRONMENT_PATH arguments disable this
behavior for the cache variables and the environment variables,
respectively, similarly to the find_package() command.
Merge topic 'FindPkgConfig_Extend-PKG_CONFIG_PATH' into next

b1dccd57 FindPkgConfig: Extend PKG_CONFIG_PATH using CMake variables
49ef91d7 FindPkgConfig: restructure documentation and document commands and variables