Revert "Keep cmake gcc builds in sync with autotools warnings."

Revert "Keep cmake gcc builds in sync with autotools warnings."

When reviewing this commit, I said

    Looks OK, although this is going to become impossible if we start
    using the externally-curated list of warnings from
    <https://www.gnu.org/software/autoconf-archive/ax_compiler_flags.html>,
    which I've been quite tempted to do.

That time has now come. I think it's more valuable to have comprehensive
warnings under our primary build system, Autotools, than to have
some fairly elaborate CMake scripting to pick up the same compiler
warnings in both build systems; the CMake build system is primarily
there to give us the ability to compile with MSVC, which has orthogonal
compiler warning options anyway.

This reverts commit 41427560af2c9923a48e50ddbf72e53aad5b2983.

Signed-off-by: Simon McVittie <smcv@debian.org>
Acked-by: Ralf Habacker <ralf.habacker@freenet.de>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=97357