Ensure that json extension is enabled when running tests.

The `commonjs_modules.phpt` test requires `json_encode` in order to
complete successfully.
Support compiling v8js with v8 in a custom path.

If you don't want to overwrite the system copy of v8 in /usr/lib/libv8.so,
you can use the --with-v8=<path> option to have php-v8js use its own
copy of libv8.
Merge pull request #35 from cscott/fixes

Makefile fixes.
The json extension is not needed for PHP 5.4.
Merge pull request #36 from cscott/more-fixes

The json extension is not needed for PHP 5.4.
Add isolate to key of template cache.
Merge pull request #37 from stesie/fix-multi-object-inst

Use v8::Isolate as part of the key to the template cache
Pass back v8 functions, don't re-wrap
Merge pull request #38 from stesie/fix-function-passback

Fix php_v8js_hash_to_jsobj for v8 functions
Create temporary HandleScope while reading properties
Create temporary HandleScope to get & call methods likewise, closes #33
Merge pull request #39 from stesie/fix-property-read

Create temporary HandleScopes when reading properties
Adapt to new v8::Persistent API, support V8 >= 3.21.12
Use v8's CopyablePersistentTraits for v8 >= 3.22.0
Remove Locker from timer thread
Merge pull request #43 from stesie/wip-ftbfs-v8-3.22

Adopt V8 API changes (up to 3.22.6)
Add a bit of documentation about g++ 4.8.
Merge pull request #44 from cscott/g++-update

Add a bit of documentation about g++ 4.8.
Ensure all v8 libraries are copied in README; sanity check in config.m4.

The latest versions of v8 build libicu as well.  If this isn't copied to
the lib directory, then the v8 version check fails (with a link error).
Ensure that this is caught at configuration time.
Fix CopyablePersistentTraits for v8 3.21.15 .. 3.21.18.x