Issue #1580: use short float repr where possible.
 - incorporate and adapt David Gay's dtoa and strtod
   into the Python core
 - on platforms where we can use Gay's code (almost
   all!), repr(float) is based on the shortest
   sequence of decimal digits that rounds correctly.
 - add sys.float_repr_style attribute to indicate
   whether we're using Gay's code or not
 - add autoconf magic to detect and enable SSE2
   instructions on x86/gcc
 - slight change to repr and str:  repr switches
   to exponential notation at 1e16 instead of
   1e17, str switches at 1e11 instead of 1e12
diff --git a/Include/pymacconfig.h b/Include/pymacconfig.h
index a8679af..7b20431 100644
--- a/Include/pymacconfig.h
+++ b/Include/pymacconfig.h
@@ -17,6 +17,9 @@
 # undef SIZEOF_VOID_P
 # undef SIZEOF__BOOL
 # undef WORDS_BIGENDIAN
+# undef DOUBLE_IS_ARM_MIXED_ENDIAN_IEEE754
+# undef DOUBLE_IS_BIG_ENDIAN_IEEE754
+# undef DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 
 #    undef VA_LIST_IS_ARRAY
 #    if defined(__LP64__) && defined(__x86_64__)
@@ -65,6 +68,9 @@
 
 #ifdef __BIG_ENDIAN__
 #define WORDS_BIGENDIAN 1
+#define DOUBLE_IS_BIG_ENDIAN_IEEE754
+#else
+#define DOUBLE_IS_LITTLE_ENDIAN_IEEE754
 #endif /* __BIG_ENDIAN */