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/FindPkgVersionWin.cmake

 # FindPkgVersion.cmake
 # --------------------
 # description:
-#   Defines the function `find_pkg_version`, which obtains the version of a
+#   Defines the function `find_pkg_version_win`, 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>)
+#   find_pkg_version_win(<library-name> <config-filename> <search-paths> <output-var-name>)
 #
 # parameters:
 #   * library-name: name of the library (eg. "cerf")
@@ -30,15 +30,15 @@
 #
 # example:
 #   * Find the version of the 'cerf' package in the `CMAKE_LIBRARY_PATH` folders:
-#     find_pkg_version("cerf" "" "${CMAKE_LIBRARY_PATH}" Cerf_VERSION)
+#     find_pkg_version_win("cerf" "" "${CMAKE_LIBRARY_PATH}" Cerf_VERSION)
 #
 # copyright: 2021 Scientific Computing Group, Forschungszentrum Jülich GmbH
 
-function(find_pkg_version libname conffile paths output_var)
+function(find_pkg_version_win libname conffile paths output_var)
 
     string(STRIP "${paths}" _paths)
     string(STRIP "${conffile}" _pkgconffile)
-    set(_msghdr "find_pkg_version::")  # message header
+    set(_msghdr "find_pkg_version_win::")  # message header
 
     # find the configuration file in the given paths
     if(NOT _pkgconffile)