NIT: Hard-coded shared object in Makefile

[GarrettVD]

Hi @zhunki

First of all, great work on this. I'd been working on and off on a syntax-aware fuzzer using this AST mutation approach myself. So, I'm glad you put in the foot-work to do this already.

Just a minor NIT here. I'd been receiving numerous Invalid Syntax errors while running afl-fuzz on an instrumented JavaScriptCore build, until I realized that the Makefile I'd built afl-fuzz with included a hard-coded reference to the PHP parser shared library. I suggest either (or both) of the following:

1. Making this parameter configurable within the Makefile with a default value
2. Updating the README.md to make it more apparent

If you'd rather I open a PR for this, let me know.