node: always build against the bundled v8 library, instead of dev-libs/v8

Rationale:

1) the v8 library bundled with node has version: 3.14.5, our v8 exheres has
version: 3.15.12. This version mismatch leads to point (2)

2) some node modules call symbols not found in the v8 3.15.12 ABI, here's a
minimal example that segfaults only when node is built against the *system*
version:

$ npm install posix-clock
$ cat test.js
var clock = require('posix-clock');
console.log(clock.gettime(clock.MONOTONIC));
$ node test.js
node: symbol lookup error: ~/node_modules/posix-clock/build/Release/posix-clock.node: undefined symbol: _ZN2v816FunctionTemplate3NewEPFNS_6HandleINS_5ValueEEERKNS_9ArgumentsEES3_NS1_INS_9SignatureEEE

3) the *only* user of dev-libs/v8 is dev-lang/node, but there is *no* gain in
disk usage when using the system v8 library.

`cave size node` -> 6.64 MBytes
`cave size v8` -> 6.33 MBytes

`cave size node` -> 11.98 MBytes

4) maintenance burden, what version of v8 should be used for the latest node
0.10.28? (our exheres is version 0.10.24) Even if we find a v8 version that
passes the node test suite, problems like the one shown in point (2) may arise.

Change-Id: I3bf539f748ac2db10015193cc4c2b40f1f3b2aaa