Remove PkgConfig dependence on Windows

PkgConfig works properly only on Linux. On Windows, it is part of the MSYS2 platform and there is no guarantee that it works properly; it is also not necessary.

Instead of using PkgConfig, a CMake module FindPkgVersionWin is introduced to find the version of a package from a config file.

find_pkg_version_win function is used to find the version of cerf package under Windows.

Related to issue #133 (closed)

cmake/commons/FindPkgVersion.cmake

-cmake_minimum_required(VERSION 3.12 FATAL_ERROR)
+# FindPkgVersion.cmake
+# --------------------
+# description:
+#   Defines the function `find_pkg_version`, which obtains the version of a
+#   library from the `pkgconfig` (*.pc) files in a given path.
+#   This is currently used only under Windows, as a simple replacement for
+#   `PkgConfig` from MSYS2 platform.
+#
+# usage:
+#   find_pkg_version(<library-name> <config-filename> <search-paths> <output-var-name>)
+#
+# parameters:
+#   * library-name: name of the library (eg. "cerf")
+#   * config-filename: name of the package config file (eg. "libcerf.pc").
+#     If an empty string is given the default will be used:
+#     'lib' + <library-name> + '.pc'
+#     eg. 'libcerf.pc'
+#   * search-paths: List of paths to search for the package config file.
+#     The file will be searched also within `pkgconfig` subfolder of the
+#     given paths.
+#   * output-var-name: Name of the output variable.
+#     If an empty string is given the default will be used:
+#     <library-name> + '_VERSION'
+#     eg. 'cerf_VERSION'
+#
+# output:
+#   The function pushes the following variables into the parent scope:
+#   * <library-name>_PATH: Path of the discovered package config file.
+#   * output-var-name: The version of the package.
+#
+# example:
+#   * Find the version of the 'cerf' package in the `CMAKE_LIBRARY_PATH` folders:
+#     find_pkg_version("cerf" "" "${CMAKE_LIBRARY_PATH}" Cerf_VERSION)
+#
+# copyright: 2021 Scientific Computing Group, Forschungszentrum Jülich GmbH
 
-# Find a package version from a configuration file
-function(findPkgVersion libname conffile paths output_var)
+function(find_pkg_version libname conffile paths output_var)
 
     string(STRIP "${paths}" _paths)
     string(STRIP "${conffile}" _pkgconffile)