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>

util/libeay.num

diff --git a/util/libeay.num b/util/libeay.num
index 36c0cd4..4c49be6 100755
--- a/util/libeay.num
+++ b/util/libeay.num
@@ -1817,3 +1817,4 @@
 sk_ASN1_OBJECT_insert                   1842
 sk_ASN1_OBJECT_push                     1843
 d2i_ASN1_SET_OF_ASN1_OBJECT             1844
+PKCS7_signatureVerify                   1845