Makefile fixes
Sorry, I have not been careful with the previous Makefile PR and made some mistakes which I here correct. I've now tested that code can be built against the installed version.
- Rename mustach.so to libmustach.so so the linker can find it
- Install headers so one can actually use the library
Also a minor change: adding -ljson-c through LDLIBS since it's needed by all linking targets.