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

PC/pyconfig.h

@@ -752,4 +752,8 @@ Py_NO_ENABLE_SHARED to find out.  Also support MS_NO_COREDLL for b/w compat */
    socket handles greater than FD_SETSIZE */
 #define Py_SOCKET_FD_CAN_BE_GE_FD_SETSIZE
 
+/* Define if C doubles are 64-bit IEEE 754 binary format, stored with the
+   least significant byte first */
+#define DOUBLE_IS_LITTLE_ENDIAN_IEEE754 1
+
 #endif /* !Py_CONFIG_H */