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>

support/dependencies/check-host-python3.mk

@@ -0,0 +1,8 @@
+# Since version 2.29, glibc requires python 3.4 or later to build the GNU C Library.
+# https://www.sourceware.org/ml/libc-alpha/2019-01/msg00723.html
+
+BR2_PYTHON3_VERSION_MIN = 3.4
+
+ifeq (,$(call suitable-host-package,python3,$(BR2_PYTHON3_VERSION_MIN) python3 python))
+BR2_PYTHON3_HOST_DEPENDENCY = host-python3
+endif

support/dependencies/check-host-python3.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+# prevent shift error
+[ $# -lt 2 ] && exit 1
+
+version_min="$(echo ${1} | awk '{ split($1, v, "."); print v[1] v[2] }')"
+
+shift
+
+# The host python interpreter is already checked by dependencies.sh but
+# it only check if the version is at least 2.7.
+# We want to check the version number of the python3 interpreter even
+# if Buildroot is able to use any version but some packages may require
+# a more recent version.
+
+for candidate in "${@}" ; do
+	python3=`which $candidate 2>/dev/null`
+	if [ ! -x "$python3" ]; then
+		continue
+	fi
+	version=`$python3 -V 2>&1 | awk '{ split($2, v, "."); print v[1] v[2] }'`
+
+	if [ $version -lt $version_min ]; then
+		# no suitable python3 found
+		continue
+	fi
+
+	# suitable python3 found
+	echo $python3
+	break
+done