Allow compiling on GCC < 5/CentOS 6 See #138.

As per https://github.com/kvspb/nginx-auth-ldap/issues/138#issuecomment-317625746
the issue is that:

* nginx compiles with `-Werror` (treating warnings as errors)
* CentOS 6 has OpenSSL 1.0.1e and so the LDAP module has a warning on compile
* CentOS 6 has GCC 4.4.  GCC treats `#pragma GCC diagnostic warning "-Wcpp"` as itself an error and thus skips the statement, leading the the `#warning` still being an error.

This adds version-checking to the warning options and the warning
itself.  This means that no warning is issued at compile time on this
type of platform, but warnings are still emitted when starting Nginx
without certificate verification, so it isn't as though the issue has
been hidden.

ngx_http_auth_ldap_module.c

@@ -41,9 +41,11 @@
 #pragma clang diagnostic warning "-W#warnings"
 #else
 #ifdef __GNUC__
+#if GNUC > 4
 #pragma GCC diagnostic warning "-Wcpp"
 #endif
 #endif
+#endif
 // TODO: do the same stuff for MSVC and/or other compilers
 
 
@@ -433,7 +435,9 @@ ngx_http_auth_ldap_ldap_server(ngx_conf_t *cf, ngx_command_t *dummy, void *conf)
       #if OPENSSL_VERSION_NUMBER >= 0x10002000
       server->ssl_check_cert = 1;
       #else
+      #if GNUC > 4
       #warning "http_auth_ldap: Compiling with OpenSSL < 1.0.2, certificate verification will be unavailable. OPENSSL_VERSION_NUMBER == " XSTR(OPENSSL_VERSION_NUMBER)
+      #endif
       ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
         "http_auth_ldap: 'ssl_cert_check': cannot verify remote certificate's domain name because "
         "your version of OpenSSL is too old. "