lib: Correctly pass collected AM_LDFLAGS to linker

AM_LDFLAGS is not automatically passed to individual library LDFLAGS
and must be passed in explicitly.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
lib: Document --enable-shared configure switch

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>