D: also look for DMD environment variable for D support.

Makefile

@@ -20,12 +20,15 @@ endif
 .PHONY: dist lint
 prereqs: optional-d-support node_modules c-preload bower_modules
 
-ifeq "" "$(shell which gdc)"
+ifneq "" "$(shell which gdc)"
 optional-d-support:
-	@echo "D language support disabled"
-else
+	$(MAKE) -C d
+else ifneq "" "$(shell which ${DMD})"
 optional-d-support:
 	$(MAKE) -C d
+else
+optional-d-support:
+	@echo "D language support disabled"
 endif
 
 NODE_MODULES=.npm-updated

d/Makefile

@@ -1,6 +1,10 @@
 GDC?=gdc
 demangle: demangle.d
+ifeq "" "$(shell which $DMD)"
+	$(DMD) -O -release $< -of$@
+else
 	$(GDC) -O2 $< -o $@
+endif
 
 clean:
 	rm -f demangle.o demangle