Honor environment linker/compiler flags

GNUmakefile

@@ -32,14 +32,14 @@ shared: shared-libs
 all: libs shared-libs test
 
 ifeq ("$(DC)","gdc")
-    DCFLAGS=-O2
-    LINKERFLAG=-Xlinker 
+    DCFLAGS?=-O2
+    LINKERFLAG?=-Xlinker
     DDOCFLAGS=-fsyntax-only -c -fdoc -fdoc-file=$@
     DDOCINC=-fdoc-inc=
     output=-o $@
 else
-    DCFLAGS=-O
-    LINKERFLAG=-L
+    DCFLAGS?=-O
+    LINKERFLAG?=-L
     DDOCFLAGS=-o- -Df$@
     output=-of$@
 endif