CPP: Make pkg-config check non-fatal.

cpp/CMakeLists.txt

@@ -50,7 +50,7 @@ endfunction (find_required_library)
 find_package (PkgConfig)
 function (check_library_version VARNAME LIBRARY_WITH_VERSION)
   if (PKG_CONFIG_FOUND)
-    pkg_check_modules (${VARNAME} REQUIRED ${LIBRARY_WITH_VERSION})
+    pkg_check_modules (${VARNAME} ${LIBRARY_WITH_VERSION})
   endif ()
 endfunction ()