Reenable -Wstrict-prototypes

One part of this patch reverts commit 22bc9a46, which disabled the
warning. The rest of it deals with the warning by adding a #pragma for
newer gcc and by disabling -Werror for compilers that can't deal with
the #pragma.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1361563731-13307-1-git-send-email-kwolf@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>