test-amalgamate

Not tested via Travis (since this is outside cmake), but at least
we can more easily validate changes to `amalgamate.py`.
diff --git a/dev.makefile b/dev.makefile
index 59aaac3..710aafe 100644
--- a/dev.makefile
+++ b/dev.makefile
@@ -24,6 +24,7 @@
 test-amalgamate:
 	python2.7 amalgamate.py
 	python3.4 amalgamate.py
+	cd dist; gcc -I. -c jsoncpp.cpp
 
 clean:
 	\rm -rf *.gz *.asc dist/