Change how SHELL is set for all calc Makefiles

Using /bin/sh on macOS due to how zsh treats globs by default.

custom/Makefile

@@ -73,6 +73,7 @@
 #	SHELL= /bin/zsh
 #
 SHELL= /bin/bash
+#SHELL= /bin/sh
 #if 0	/* start of skip for non-Gnu makefiles */
 ifeq ($(target),Darwin)
 SHELL:= /bin/zsh