modules: do not include gmodule-2.0 in static builds

gmodule-2.0's pkg-config files include -Wl,--export-dynamic, which breaks
static builds.  It is a glib bug, but we need to support --static builds for
the linux-user targets, and in the end all that is needed to fix this is:

* outlaw --enable-modules --static, which makes little sense anyway

* only include gmodule-2.0's cflags and ldflags if --enable-modules is
specified on the command line.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1393346215-5636-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>