gnutls: bump lowest supported version to 3.1.10

GnuTLS 3.1.10 added new functions we want to use. That version was
released on Mar 22, 2013. Removing support for older versions also
greatly simplifies the code.

Ref: #5271
Closes #5276

configure.ac

@@ -2084,7 +2084,8 @@ if test -z "$ssl_backends" -o "x$OPT_GNUTLS" != xno; then
          CPPFLAGS="$CPPFLAGS $addcflags"
       fi
 
-      AC_CHECK_LIB(gnutls, gnutls_check_version,
+      dnl this function is selected since it was introduced in 3.1.10
+      AC_CHECK_LIB(gnutls, gnutls_x509_crt_get_dn2,
        [
        AC_DEFINE(USE_GNUTLS, 1, [if GnuTLS is enabled])
        AC_SUBST(USE_GNUTLS, [1])
@@ -2112,7 +2113,6 @@ if test -z "$ssl_backends" -o "x$OPT_GNUTLS" != xno; then
             AC_MSG_NOTICE([Added $gtlslib to CURL_LIBRARY_PATH])
           fi
         fi
-        AC_CHECK_FUNCS([gnutls_certificate_set_x509_key_file2 gnutls_alpn_set_protocols gnutls_ocsp_req_init])
       fi
 
     fi

docs/INTERNALS.md

@@ -85,7 +85,7 @@ Dependencies
 ------------
 
  - OpenSSL      0.9.7
- - GnuTLS       2.11.3
+ - GnuTLS       3.1.10
  - zlib         1.1.4
  - libssh2      0.16
  - c-ares       1.6.0