https://bugs.gentoo.org/831305

Upstream git has merged autotoolsification PR so no need for this.
--- a/pppd/Makefile.linux
+++ b/pppd/Makefile.linux
@@ -155,7 +155,7 @@ endif
 
 ifdef NEEDDES
 ifndef USE_CRYPT
-CFLAGS   += -I$(shell $(CC) --print-sysroot)/usr/include/openssl
+CFLAGS   += -I$(INSTROOT)/usr/include/openssl
 NEEDCRYPTOLIB = y
 else
 CFLAGS   += -DUSE_CRYPT=1