dev-python/pythran: add comment explaining an inscrutable sed

It's not obvious why this change needs to be made in installed code, and
in particular why it is test-only.

I'm *still* not sure why this is the best change, but at least with a
link people can follow the logic when investigating.

Bug: https://bugs.gentoo.org/916461
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>
Signed-off-by: Sam James <sam@gentoo.org>

dev-python/pythran/pythran-0.15.0-r1.ebuild

@@ -61,6 +61,7 @@ src_configure() {
 	rm -r pythran/{boost,xsimd} || die
 
 	if use test ; then
+		# https://bugs.gentoo.org/916461
 		sed -i \
 			-e 's|blas=blas|blas=cblas|' \
 			-e 's|libs=|libs=cblas|' \