configure: Make missing pkg-config an error rather than a warning

configure: Make missing pkg-config an error rather than a warning

If pkg-config doesn't exist then make configure fail immediately
with a useful error message. Now that glib is a required dependency,
proceeding despite the missing pkg-config will just cause us to
fail later with a misleading message about glib not being present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>