update BSDmakefile to latest version from upstream

BSDmakefile

@@ -1,6 +1,7 @@
 # GNU makefile proxy script for BSD make
+#
 # Written and maintained by Mahmoud Al-Qudsi <[email protected]>
-# Copyright NeoSmart Technologies <https://neosmart.net/> 2014-2018
+# Copyright NeoSmart Technologies <https://neosmart.net/> 2014-2019
 # Obtain updates from <https://github.com/neosmart/gmake-proxy>
 #
 # Redistribution and use in source and binary forms, with or without
@@ -26,26 +27,32 @@
 
 JARG =
 GMAKE = "gmake"
-#When gmake is called from another make instance, -w is automatically added
-#which causes extraneous messages about directory changes to be emitted.
-#--no-print-directory silences these messages.
+# When gmake is called from another make instance, -w is automatically added
+# which causes extraneous messages about directory changes to be emitted.
+# Running with --no-print-directory silences these messages.
 GARGS = "--no-print-directory"
 
 .if "$(.MAKE.JOBS)" != ""
-JARG = -j$(.MAKE.JOBS)
+    JARG = -j$(.MAKE.JOBS)
 .endif
 
-#by default bmake will cd into ./obj first
+# bmake prefers out-of-source builds and tries to cd into ./obj (among others)
+# where possible. GNU Make doesn't, so override that value.
 .OBJDIR: ./
 
+# The GNU convention is to use the lowercased `prefix` variable/macro to
+# specify the installation directory. Humor them.
+GPREFIX = ""
+.if defined(PREFIX) && ! defined(prefix)
+    GPREFIX = 'prefix = "$(PREFIX)"'
+.endif
+
+.BEGIN: .SILENT
+	which $(GMAKE) || printf "Error: GNU Make is required!\n\n" 1>&2 && false
+
 .PHONY: FRC
 $(.TARGETS): FRC
-	$(GMAKE) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)
+	$(GMAKE) $(GPREFIX) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)
 
 .DONE .DEFAULT: .SILENT
-	$(GMAKE) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)
-
-.ERROR: .SILENT
-	if ! which $(GMAKE) > /dev/null; then \
-		echo "GNU Make is required!"; \
-	fi
+	$(GMAKE) $(GPREFIX) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)