Strip with $(STRIP), not the build host strip.

diff -Nrup a/Makefile b/Makefile
--- a/Makefile	2020-12-07 23:12:51.000000000 +0100
+++ b/Makefile	2026-10-22 17:22:16.983301867 +0200
@@ -6,7 +6,7 @@ doc: ssss.1 ssss.1.html
 
 ssss-split: ssss.c
 	$(CC) -W -Wall -O2 -o ssss-split ssss.c -lgmp
-	strip ssss-split
+	$(STRIP) ssss-split
 
 ssss-combine: ssss-split
 	ln -f ssss-split ssss-combine