configure: integrate Meson in the build system

The Meson build system is integrated in the existing configure/make steps
by invoking Meson from the configure script and converting Meson's build.ninja
rules to an included Makefile.

build.ninja already provides tags/ctags/cscope rules, so they are removed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>