CMake: Add a CMake module 'FindPkgVersion' to find the version of a package from a config file

This module is a simple replacement for `PkgConfig`.

cmake/find/FindPkgVersion.cmake

+cmake_minimum_required(VERSION 3.12 FATAL_ERROR)
+
+# Find a package version from a configuration file
+function(findPkgVersion libname conffile paths output_var)
+
+  string(STRIP "${paths}" _paths)
+  string(STRIP "${conffile}" _pkgconffile)
+
+  # find the configuration file in the given paths
+  if(NOT _pkgconffile)
+    set(_pkgconffile "lib${libname}.pc")
+  endif()
+
+  # initialize the output variable
+  if(NOT output_var)
+    set(_outvar ${libname}_VERSION)
+  else()
+    set(_outvar ${output_var})
+  endif()
+  set(${_outvar} "${_outvar}-NOTFOUND" PARENT_SCOPE)
+
+  set(_path_sfx pkgconfig)  # search subdirectories
+
+  # NOTE: ${libname}_PATH variable will be cached
+  find_path("${libname}_PATH" "${_pkgconffile}"
+    PATHS "${_paths}" PATH_SUFFIXES "${_path_sfx}"
+    DOC "Package configuration for ${_pkgconffile}")
+
+  # NOTE:
+  # A CMake value is False if the constant is 0, OFF, NO, FALSE,
+  # N, IGNORE, "", or ends in the suffix '-NOTFOUND'.
+  if(${libname}_PATH)
+    message(STATUS "'${_pkgconffile}' found in '${${libname}_PATH}'")
+  else()
+    message(WARNING "'${_pkgconffile}' not found in paths {${_paths}}")
+    return()
+  endif()
+
+  # extract the configuration information
+  file(READ "${${libname}_PATH}/${_pkgconffile}" _conf)
+  string(TOLOWER "${_conf}" _conf_lc)
+  string(REGEX MATCH "version[: \t]*([1-9.]+)" _rxmatch ${_conf_lc})
+
+  if(${CMAKE_MATCH_1})
+    set(${_outvar} "${CMAKE_MATCH_1}" PARENT_SCOPE)
+  endif ()
+
+endfunction()