Merge pull request #44 from cscott/g++-update

Add a bit of documentation about g++ 4.8.

README.md

@@ -39,6 +39,14 @@ sudo cp include/v8* /usr/include
 
 ```
 
+`v8` doesn't support `g++` version 4.8 (yet; see
+<https://code.google.com/p/v8/issues/detail?id=2149>).  If your default `g++`
+is version 4.8 you may need to install `g++` 4.7 and replace the `make`
+command above with:
+```
+CXX=g++-4.7 LINK=g++-4.7 make native library=shared -j8
+```
+
 If you don't want to overwrite the system copy of v8, replace `/usr` in
 the above commands with `/tmp/v8-install` and then add
 `--with-v8js=/tmp/v8-install` to the php-v8js `./configure` command below.