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

-cmake_minimum_required(VERSION 3.12 FATAL_ERROR)
+# FindPkgVersion.cmake
+# --------------------
+# description:
+#   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_win(<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_win("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_win libname conffile paths output_var)
 
     string(STRIP "${paths}" _paths)
     string(STRIP "${conffile}" _pkgconffile)
+    set(_msghdr "find_pkg_version_win::")  # message header
 
     # find the configuration file in the given paths
     if(NOT _pkgconffile)
@@ -12,10 +46,9 @@ function(findPkgVersion libname conffile paths output_var)
     endif()
 
     # initialize the output variable
-    if(NOT output_var)
+    set(_outvar ${output_var})
+    if(NOT _outvar)
         set(_outvar ${libname}_VERSION)
-    else()
-        set(_outvar ${output_var})
     endif()
     set(${_outvar} "${_outvar}-NOTFOUND" PARENT_SCOPE)
 
@@ -26,13 +59,10 @@ function(findPkgVersion libname conffile paths output_var)
         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}'")
+        message(STATUS "${_msghdr} '${_pkgconffile}' found in '${${libname}_PATH}'")
     else()
-        message(WARNING "'${_pkgconffile}' not found in paths {${_paths}}")
+        message(WARNING "${_msghdr} '${_pkgconffile}' not found in paths {${_paths}}")
         return()
     endif()
 
@@ -41,8 +71,11 @@ function(findPkgVersion libname conffile paths output_var)
     string(TOLOWER "${_conf}" _conf_lc)
     string(REGEX MATCH "version[: \t]*([1-9.]+)" _rxmatch ${_conf_lc})
 
-    if(${CMAKE_MATCH_1})
+    if(CMAKE_MATCH_1)
         set(${_outvar} "${CMAKE_MATCH_1}" PARENT_SCOPE)
+        message(STATUS "${_msghdr} ${_outvar} => ${CMAKE_MATCH_1}")
+    else()
+        message(WARNING "${_msghdr} ${_outvar} not found")
     endif ()
 
 endfunction()