configure: Fix tests for compiler warning flags.

configure: Fix tests for compiler warning flags.

`AC_LANG_PROGRAM` ignores compiler warnings.  This behaviour made the tests
always true and thus without any effect.  We now add `-Werror` to enforce an
error in case the warning flag is not known.

To make that work it is also necessary to compile `confdefs.h` without
warnings; we add a hack to undefine macro `PACKAGE_NAME`, which gets defined
twice for historical reasons.

[hahnjo, 2019-11-24 11:44:12]

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.)

[lemzwerg, 2019-11-24 15:11:24]

> 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.)

I agree.  My patch is rather a proof of concept to show what's necessary. 
However, this 'untangling' is a tedious thing, and I favour a quick 'n' dirty
solution over something that needs a lot of thinking and will happen in the far,
far future...  For example, we could mark is as a hack even more prominently
than the 'urg' notice.  But maybe you are working on this right now :-)

[lemzwerg, 2019-11-30 14:42:39]

Only change `confdefs.h`; the rest is already handled by issue #5608.