check for MirBSD ksh

Makefile

@@ -153,6 +153,10 @@ NO_ERROR=0
 		echo "GNU bash needs to be installed."; \
 		exit 1; \
 	fi
+	@if ! mksh -c 'echo $$KSH_VERSION' 2>&1 | fgrep 'MIRBSD' >/dev/null 2>&1; then \
+		echo "MirBSD ksh (mksh) needs to be installed."; \
+		exit 1; \
+	fi
 	@if test x"$$(umask 2>/dev/null | sed 's/00*22/OK/')" != x"OK"; then \
 		echo >&2 Error: you must build with umask 022, sorry.; \
 		exit 1; \