enable cross-compiling
diff --git a/src/Makefile.am b/src/Makefile.am
index 260834d..3e32c25 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -206,6 +206,14 @@
BUILT_SOURCES = $(protoc_outputs)
+if USE_EXTERNAL_PROTOC
+
+unittest_proto_middleman: $(protoc_inputs)
+ $(PROTOC) -I$(srcdir) --cpp_out=. $(protoc_inputs)
+ touch unittest_proto_middleman
+
+else
+
# This rule is a little weird. The first prereq is the protoc executable
# and the rest are its inputs. Therefore, $^ -- which expands to the
# list of prereqs -- is actually a valid command. We have to place "./" in
@@ -220,6 +228,8 @@
./$^ -I$(srcdir) --cpp_out=.
touch unittest_proto_middleman
+endif
+
$(protoc_outputs): unittest_proto_middleman
COMMON_TEST_SOURCES = \
