meson/configure: add 'valgrind' option & --{en, dis}able-valgrind flag

meson/configure: add 'valgrind' option & --{en, dis}able-valgrind flag

Currently valgrind debugging support for coroutine stacks is enabled
unconditionally when valgrind/valgrind.h is found. There is no way
to disable valgrind support if valgrind.h is present in the build env.

This is bad for distros, as an dependency far down the chain may cause
valgrind.h to become installed, inadvertently enabling QEMU's valgrind
debugging support. It also means if a distro wants valgrind support
there is no way to mandate this.

The solution is to add a 'valgrind' build feature to meson and thus
configure script.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250425121713.1913424-1-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>