Build: Detect supported compiler warning flags better.

Clang and nowadays also GCC accept any -Wfoobar option
but then may give a warning that an unknown warning option
was specified. To avoid adding unsupported warning options,
the options are now tested with -Werror.

Thanks to Charles Diza.
xzgrep: Avoid passing both -q and -l to grep.

The behavior of grep -ql varies:
  - GNU grep behaves like grep -q.
  - OpenBSD grep behaves like grep -l.

POSIX doesn't make it 100 % clear what behavior is expected.
Anyway, using both -q and -l at the same time makes no sense
so both options simply should never be used at the same time.

Thanks to Christian Weisgerber.
Add support for AmigaOS/AROS to tuklib_physmem().

Thanks to Fredrik Wikstrom.
Update THANKS.
Build: Fix Autoconf warnings about escaped backquotes.
Build: Replace obsolete AC_HELP_STRING with AS_HELP_STRING.
Build: Update m4/ax_pthread.m4 from Autoconf Archive.
Update INSTALL about a "make check" failure in test_scripts.sh.
Update THANKS.
Translations: Update the French translation.

Thanks to Adrien Nader.
Update NEWS for 5.0.8.
Bump version and soname for 5.0.8.
xz: Fix a comment.
po/fr: missing line in translation of --extreme.
po/fr: improve wording for help for --lzma1/--lzma2.