New function PKCS7_signatureVerify to allow the signing certificate to

be explicitly stated with PKCS#7 verify.

Also fix for util/mkerr.pl: if the -nostatic option is being used this will be
for an external library so the autogenerated C file should include the
header file as:
#include "any/path/to/header.h"
rather than the internal library form:
#include <openssl/header.h>