Make the Makefile respect the environment flags Debian provides,
to fix builds.
--- a/Makefile
+++ b/Makefile
@@ -2,8 +2,9 @@
 #
 #   make sharedlibrary  : make shared library
 
-D_COMPILER=ldc2
-DFLAGS = -wi -g -relocation-model=pic -Icontrib/undead -L-lz
+DC ?= ldc2
+DFLAGS ?= -wi -g -relocation-model=pic
+DFLAGS += -Icontrib/undead -L-lz
 
 ifndef GUIX
   ifdef GUIX_ENVIRONMENT
@@ -34,11 +35,11 @@
 default debug release static sharedlibrary: $(BIN)
 
 %.o: %.d
-	$(D_COMPILER) $(DFLAGS) -c $< -od=$(dir $@)
+	$(DC) $(DFLAGS) -c $< -od=$(dir $@)
 
 $(BIN): $(OBJ)
 	$(info linking...)
-	$(D_COMPILER) -main $(DFLAGS) $(OBJ) -of=$(BIN)
+	$(DC) -main $(DFLAGS) $(OBJ) -of=$(BIN)
 
 check: $(BIN)
 	$(info running tests...)