#5608 Avoid easy warnings with Clang

[Jonas Hahnfeld]

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.

1. 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...

http://codereview.appspot.com/573270043

[Anonymous]

Passes make, make test-baseline and a full make doc

[Jonas Hahnfeld]

Fix comment

http://codereview.appspot.com/573270043

[Anonymous]

Passes make, make test-baseline and a full make doc

[Anonymous]

Patch on countdown for Nov 25th

[Anonymous]

Patch counted down - please push.

[Jonas Hahnfeld]

Add comment about GCC

http://codereview.appspot.com/573270043

[Jonas Hahnfeld]

Pushed to staging as:

commit 9926ad3e43ab99c7463ca646fd4321647a9d72da
Author: Jonas Hahnfeld <hahnjo@hahnjo.de>
Date:   Tue Nov 19 18:56:26 2019 +0100

    Issue 5608/2: 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...

commit 0b62028523d8d81cbc81b48ce10d14d9f680510c
Author: Jonas Hahnfeld <hahnjo@hahnjo.de>
Date:   Tue Nov 19 18:55:29 2019 +0100

    Issue 5608/1: 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.