mercurial-native: fix install

else host python is used leading to the following error if the host doesn't
have the python headers installed :
| Python headers are required to build Mercurial
| make: *** [build] Error 1

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>