Add support for pkgconfig for libopenvswitch
* Add automake condition that will be set when --enable-shared is
  specified
* Add rule to generate pkgconfig .pc file from configure
* Install pkg-config file to $(libdir)/pkgconfig

Signed-off-by: Rob Adams <readams@readams.net>
Add missing ofpbuf.h to headers
Merge pull request #1 from readams/pkgconfig

Add support for pkgconfig for libopenvswitch