diff --git a/src/build.rules b/src/build.rules
index bdcae6551..32f45ae1e 100644
--- a/src/build.rules
+++ b/src/build.rules
@@ -7,7 +7,8 @@ all: _all
 # setup some variables
 ROOTDIR := $(dir $(lastword $(MAKEFILE_LIST)))
 ROOTDIR := $(dir $(ROOTDIR:%../src/=%))../
-BUILDDIR := $(ROOTDIR)build
+BUILDDIR ?= $(abspath $(ROOTDIR)build)
+BUILDDIR := $(BUILDDIR:%/=%)
 _PROJ := $(abspath $(dir $(firstword $(MAKEFILE_LIST))))
 ABSROOT := $(abspath $(ROOTDIR))
 _PROJ := $(_PROJ:$(ABSROOT)/%=%)