support/dependencies: add a check for python3

support/dependencies: add a check for python3

Since version 2.29, glibc requires python 3.4 or later to build the
GNU C Library [1].

We add a new check to verify the version of python3 interpreter
installed on the host.  If no suitable python3 interpreter is found,
define BR2_PYTHON3_HOST_DEPENDENCY to add host-python3 in package
dependencies when needed.

[1] https://www.sourceware.org/ml/libc-alpha/2019-01/msg00723.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
Reviewed-by: Adam Duskett <aduskett@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: drop not so useful comment in the .mk file, as suggested by
Yann E. Morin.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>