Remove PkgConfig dependence on Windows

`PkgConfig` works properly only on Linux. On Windows, there is no guarantee that
it works properly. It is also not necessary.

Related to issue #133

cmake/find/FindCerf.cmake

@@ -16,12 +16,17 @@ set(Cerf_ULTIMATELY_REQUIRED ${Cerf_FIND_REQUIRED})
 set(Cerf_FIND_REQUIRED FALSE)
 # set(Cerf_FIND_VERSION 1.13) incompatible with Debian package
 
-find_package(PkgConfig QUIET)
-if(PKG_CONFIG_FOUND)
+# Do not rely on PkgConfig on Windows
+if(NOT WIN32)
+  find_package(PkgConfig QUIET)
+  if(PKG_CONFIG_FOUND)
     pkg_check_modules(PC_Cerf QUIET libcerf)
     if(PC_Cerf_FOUND)
-        set(Cerf_VERSION ${PC_Cerf_VERSION})
+      set(Cerf_VERSION ${PC_Cerf_VERSION})
     endif()
+  endif()
+else()
+  set(Cerf_VERSION "")
 endif()
 
 find_path(Cerf_INCLUDE_DIR cerf.h)