meson: leave unnecessary modules out of the build

meson: leave unnecessary modules out of the build

meson.build files choose whether to build modules based on foo.found()
expressions.  If a feature is enabled (e.g. --enable-gtk), these expressions
are true even if the code is not used by any emulator, and this results
in an unexpected difference between modular and non-modular builds.

For non-modular builds, the files are not included in any binary, and
therefore the source files are never processed.  For modular builds,
however, all .so files are unconditionally built by default, and therefore
a normal "make" tries to build them.  However, the corresponding trace-*.h
files are absent due to this conditional:

if have_system
  trace_events_subdirs += [
    ...
    'ui',
    ...
  ]
endif

which was added to avoid wasting time running tracetool on unused trace-events
files.  This causes a compilation failure; fix it by skipping module builds
entirely if (depending on the module directory) have_block or have_system
are false.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>