only use the SSPI support with internal neon

neon 0.30.0 has added support for SSPI (author of the commit is kso,
which sounds familiar :-), so NE_FEATURE_SSPI is defined, but the
signature of ne_auth_creds remains the same as before. That means that
build with system neon 0.30.0 fails...

RepositoryExternal.mk

@@ -867,6 +867,7 @@ ifeq ($(SYSTEM_NEON),YES)
 define gb_LinkTarget__use_neon
 $(call gb_LinkTarget_add_defs,$(1),\
 	-DNEON_VERSION=0x$(NEON_VERSION) \
+	-DSYSTEM_NEON \
 )
 $(call gb_LinkTarget_set_include,$(1),\
 	$$(INCLUDE) \

ucb/source/ucp/webdav-neon/NeonSession.cxx

@@ -228,7 +228,7 @@ extern "C" int NeonSession_ResponseBlockWriter( void * inUserData,
 }
 
 extern "C" int NeonSession_NeonAuth( void *       inUserData,
-#ifdef NE_FEATURE_SSPI
+#if defined NE_FEATURE_SSPI && ! defined SYSTEM_NEON
                                      const char * inAuthProtocol,
 #endif
                                      const char * inRealm,
@@ -297,7 +297,7 @@ extern "C" int NeonSession_NeonAuth( void *       inUserData,
 
     bool bCanUseSystemCreds = false;
 
-#ifdef NE_FEATURE_SSPI
+#if defined NE_FEATURE_SSPI && ! defined SYSTEM_NEON
     bCanUseSystemCreds
         = (attempt == 0) && // avoid endless loops
           ne_has_support( NE_FEATURE_SSPI ) && // Windows-only feature.