net-dialup/mgetty: fix tests to run on posix

Closes: https://bugs.gentoo.org/889744
Signed-off-by: Conrad Kostecki <conikost@gentoo.org>

net-dialup/mgetty/mgetty-1.2.1-r4.ebuild

@@ -1,4 +1,4 @@
-# Copyright 1999-2022 Gentoo Authors
+# Copyright 1999-2024 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=8
@@ -65,6 +65,9 @@ src_prepare() {
 		-e 's:^CFLAGS=:CFLAGS+= -I..:g' \
 		-e 's:^RANLIB=:RANLIB?=:g' \
 		*/Makefile || die
+
+	# Use POSIX compatible commands
+	sed -e 's/echo -e/printf %b/g' -i  Makefile || die
 }
 
 src_configure() {