Fix cross-compiling when building in a dedicated build directory.  Fix pointed out by Pierre Kestener.

src/Makefile.am

diff --git a/src/Makefile.am b/src/Makefile.am
index f009838..2b1d6da 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -234,7 +234,7 @@
 if USE_EXTERNAL_PROTOC
 
 unittest_proto_middleman: $(protoc_inputs)
-	$(PROTOC) -I$(srcdir) --cpp_out=. $(protoc_inputs)
+	$(PROTOC) -I$(srcdir) --cpp_out=. $^
 	touch unittest_proto_middleman
 
 else