apibuild: Generate macro/@string attribute

apibuild: Generate macro/@string attribute

If a macro has a string value, the @string attribute will contain the
value. Otherwise @string attribute will be missing.

For example, the following macro definition from libvirt-domain.h:

 /**
  * VIR_MIGRATE_PARAM_URI:
  * ...
  */
 # define VIR_MIGRATE_PARAM_URI               "migrate_uri"

will result in

 <macro name='VIR_MIGRATE_PARAM_URI' file='libvirt-domain' string='migrate_uri'>
   <info><![CDATA[...]]></info>
 </macro>

https://bugzilla.redhat.com/show_bug.cgi?id=1229199

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>