Avoid easy warnings with Clang

Avoid easy warnings with Clang

Building with Clang is currently a lot nosier than with GCC. This
represents a first step into the right direction by picking the
low-hanging fruits.

1. Fix warning flag detection for Clang

Clang requires -Werror=unknown-warning-option to actually throw an
error if it doesn't know about a warning flag.

2. Fix struct vs. class template for PQueue

Clang issues a warning that this could be an issue for the
"Windows C++ ABI". Apparently, that's not really causing problems
as the mismatch was introduced by commit c8989177a in 1997...

[lemzwerg, 2019-11-19 22:13:40]

LGTM

https://codereview.appspot.com/573270043/diff/549210045/configure.ac
File configure.ac (right):

https://codereview.appspot.com/573270043/diff/549210045/configure.ac#newcode212
configure.ac:212: # options. Unfortunately GCC doesn't support this options, so
we just
s/options/option/

And please use two spaces after a sentence stop.

[hahnjo, 2019-11-20 12:28:00]

Fix comment

[lemzwerg, 2019-11-24 15:13:56]

Please extend this patch to cover gcc, too.

[hahnjo, 2019-11-24 17:15:45]

On 2019/11/24 15:13:56, lemzwerg wrote:
> Please extend this patch to cover gcc, too.

AFAICT this already works with GCC because it just returns an error code:
 $ gcc -Wnot-there -x c /dev/null 
gcc: error: unrecognized command line option ‘-Wnot-there’
 $ echo $?
1

[lemzwerg, 2019-11-24 17:24:07]

> AFAICT this already works with GCC because it just
> returns an error code: [...]

OK.  This deserves a comment IMHO.

[hahnjo, 2019-11-24 17:27:06]

On 2019/11/24 17:24:07, lemzwerg wrote:
> > AFAICT this already works with GCC because it just
> > returns an error code: [...]
> 
> OK.  This deserves a comment IMHO.

Ok, will add a description of the status-quo for GCC.

[hahnjo, 2019-11-25 17:34:31]

Add comment about GCC