[cmake,bugfix] Allow 0 as a version.

Previously we checked whether the extracted Version evaluated to
True. Unfortunately, there lazy people like me that like use a version
of 0 for testing, which evaluates to False and breaks configure.
With this patch we allow 0 to and just test wether the version consists
of characters.

cmake/modules/DuneMacros.cmake

@@ -186,9 +186,9 @@ macro(dune_module_information MODULE_DIR)
 
   # find version strings
   extract_line("Version:" MODULE_LINE "${DUNE_MODULE}")
-  if(NOT MODULE_LINE)
+  if(NOT MODULE_LINE MATCHES ".+")
     message(FATAL_ERROR "${MODULE_DIR}/dune.module is missing a version.")
-  endif(NOT MODULE_LINE)
+  endif(NOT MODULE_LINE MATCHES ".+")
 
   string(REGEX REPLACE ".*Version:[ ]*([^ \n]+).*" "\\1" DUNE_MOD_VERSION "${MODULE_LINE}")
   string(REGEX REPLACE "([0-9]).*" "\\1" DUNE_VERSION_MAJOR "${DUNE_MOD_VERSION}")