python3: optional decimal module support

This commit makes the decimal module support optional: since Python
3.3 it relies on an external library. The source code of this library
is available built-in as part of the Python sources, but in Buildroot
we generally prefer to use the external library when possible.

To achieve this, this commit adds a patch to Python that is similar to
the one we use for expat support, but this time for the
libmpdec/mpdecimal library.

As a consequence, since mpdecimal now builds properly even when
<fenv.h> is not available (on i386), this commit fixes:

  http://autobuild.buildroot.org/results/b64/b64d5c941a7cac00619da3a0696939f86a8eafc2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>