Don't warn when not finding an option starting with _opt_

This happens for e.g. _opt_CC and friends and is allowed to be empty/unset
according to the code in the setter function.
This avoids half a dozen warnings per software part installed

Author: Flamefire

easybuild/tools/toolchain/options.py

@@ -86,7 +86,12 @@ def option(self, name, templatedict=None):
         """Return option value"""
         value = self.get(name, None)
         if value is None and name not in self.options_map:
-            self.log.warning("option: option with name %s returns None" % name)
+            msg = "option: option with name %s returns None" % name
+            # Empty options starting with _opt_ are allowed, so don't warn
+            if name.startswith('_opt_'):
+                self.log.devel(msg)
+            else:
+                self.log.warning(msg)
             res = None
         elif name in self.options_map:
             res = self.options_map[name]