Use a trick to make the test for GMP v2 to work when GMP v1 defines
__GNU_MP__ as empty: #if __GNU_MP__ + 0 == 2
(Untested.)
diff --git a/Modules/mpzmodule.c b/Modules/mpzmodule.c
index a2bcfed..bfa8ff7 100644
--- a/Modules/mpzmodule.c
+++ b/Modules/mpzmodule.c
@@ -94,7 +94,7 @@
 #include "gmp.h"
 #include "gmp-impl.h"
 
-#if __GNU_MP__ == 2
+#if __GNU_MP__ + 0 == 2
 #define GMP2
 #else
 #define MPZ_GET_STR_BUG