eclass/distutils-r1: Sync with Gentoo

It's from Gentoo commit 3f87d0f277763f1cd621a78e34eb226d89020863.

sdk_container/src/third_party/portage-stable/eclass/distutils-r1.eclass

@@ -50,6 +50,7 @@ case ${EAPI} in
 esac
 
 # @ECLASS_VARIABLE: DISTUTILS_EXT
+# @PRE_INHERIT
 # @DEFAULT_UNSET
 # @DESCRIPTION:
 # Set this variable to a non-null value if the package (possibly
@@ -1242,7 +1243,7 @@ _distutils-r1_get_backend() {
 	if [[ -f pyproject.toml ]]; then
 		# if pyproject.toml exists, try getting the backend from it
 		# NB: this could fail if pyproject.toml doesn't list one
-		build_backend=$(gpep517 get-backend)
+		build_backend=$("${EPYTHON}" -m gpep517 get-backend)
 	fi
 	if [[ -z ${build_backend} && ${DISTUTILS_USE_PEP517} == setuptools &&
 		-f setup.py ]]
@@ -1316,7 +1317,7 @@ distutils_wheel_install() {
 
 	einfo "  Installing ${wheel##*/} to ${root}"
 	local cmd=(
-		gpep517 install-wheel
+		"${EPYTHON}" -m gpep517 install-wheel
 			--destdir="${root}"
 			--interpreter="${PYTHON}"
 			--prefix="${EPREFIX}/usr"
@@ -1445,7 +1446,7 @@ distutils_pep517_install() {
 	local build_backend=$(_distutils-r1_get_backend)
 	einfo "  Building the wheel for ${PWD#${WORKDIR}/} via ${build_backend}"
 	local cmd=(
-		gpep517 build-wheel
+		"${EPYTHON}" -m gpep517 build-wheel
 			--prefix="${EPREFIX}/usr"
 			--backend "${build_backend}"
 			--output-fd 3
@@ -1806,6 +1807,11 @@ distutils-r1_run_phase() {
 	tc-export AR CC CPP CXX
 
 	if [[ ${DISTUTILS_EXT} ]]; then
+		if [[ ${BDEPEND} == *dev-python/cython* ]] ; then
+			# Workaround for https://github.com/cython/cython/issues/2747 (bug #918983)
+			local -x CFLAGS="${CFLAGS} $(test-flags-CC -Wno-error=incompatible-pointer-types)"
+		fi
+
 		local -x CPPFLAGS="${CPPFLAGS} $(usex debug '-UNDEBUG' '-DNDEBUG')"
 		# always generate .c files from .pyx files to ensure we get latest
 		# bug fixes from Cython (this works only when setup.py is using