The C standard leaves it up to the implementation to determine the sign of
bit fields when left unspecified.  Python assumes the compiler will
default to signed; however, this is not the case for Solaris and AIX.
Therefore, we should explicitly declare the sign of bit fields where
applicable.

--- a/Modules/_ctypes/_ctypes_test.c
+++ b/Modules/_ctypes/_ctypes_test.c
@@ -370,8 +370,8 @@
 EXPORT(unsigned PY_LONG_LONG) last_tf_arg_u;
 
 struct BITS {
-    int A: 1, B:2, C:3, D:4, E: 5, F: 6, G: 7, H: 8, I: 9;
-    short M: 1, N: 2, O: 3, P: 4, Q: 5, R: 6, S: 7;
+    signed int A: 1, B:2, C:3, D:4, E: 5, F: 6, G: 7, H: 8, I: 9;
+    signed M: 1, N: 2, O: 3, P: 4, Q: 5, R: 6, S: 7;
 };
 
 EXPORT(void) set_bitfields(struct BITS *bits, char name, int value)