python3: fix double format detection

Python is not able to detect if compiler double representation is
compliant with IEE754:

  checking whether C doubles are little-endian IEEE 754 binary64... no
  checking whether C doubles are big-endian IEEE 754 binary64... no
  checking whether C doubles are ARM mixed-endian IEEE 754 binary64... no

Accordingly 'legacy' mode isused. It is possible to check this at
runtime by check if 'sys.float_repr_style' contains 'short' or
'legacy'. Calculus correctness is not garanteed with 'legacy'.

Problem is better described here:

  http://stackoverflow.com/questions/29920294/what-causes-pythons-float-repr-style-to-use-legacy
  https://bugs.python.org/issue7117

However, all gcc architecture use a representation compliant with
IEE754. So, we can enable it unconditionnaly.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
[Thomas: rework condition to not use strip, as suggested by Baruch.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/python3/python3.mk

@@ -126,6 +126,13 @@ PYTHON3_CONF_ENV += \
 	ac_cv_working_tzset=yes \
 	ac_cv_prog_HAS_HG=/bin/false
 
+# GCC is always compliant with IEEE754
+ifeq ($(BR2_ENDIAN),"LITTLE")
+PYTHON3_CONF_ENV += ac_cv_little_endian_double=yes
+else
+PYTHON3_CONF_ENV += ac_cv_big_endian_double=yes
+endif
+
 # uClibc is known to have a broken wcsftime() implementation, so tell
 # Python 3 to fall back to strftime() instead.
 ifeq ($(BR2_TOOLCHAIN_USES_UCLIBC),y)