Clarify requirement for GNU make
diff --git a/Doc/Makefile b/Doc/Makefile
index 852bf50..a17ad81 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -83,6 +83,8 @@
# $(shell) function here to work around that restriction by
# identifying mkhowto and the commontex/ directory using absolute paths.
#
+# If your doc build fails immediately, you may need to switch to GNU make.
+# (e.g. OpenBSD needs package gmake installed; use gmake instead of make)
PWD=$(shell pwd)
# (The trailing colon in the value is needed; TeX places it's default