configure: Fix tests for compiler warning flags. (issue 551210043 by lemzwerg@googlemail.com)

[jonas . hahnfeld]

I don't think we should go this hacky way. For Clang I'm already fixing
this issue with https://sourceforge.net/p/testlilyissues/issues/5608/

(Especially the confdefs.h should be fixed correctly, by untangling
AutoConf and StepMake and not define the macro twice in the first
place.)

https://codereview.appspot.com/551210043/