Changes to convert_js.py to support gyp restrictions

Changes to convert_js.py to support gyp restrictions.

This needs testing on Linux and MacOS, not just looking at the text. The changes to libadblockplus.gyp were extracted by hand from my personal modifications for Windows (on an as-yet secret branch on my local repository), because libadblockplus.gyp as it is now is still failing spectacularly on Windows. It does work, however, on my modified version, which has been tested only on Windows.

[Eric, 2013-04-19 12:52:44]

I should have hit this button on the review UI yesterday, apparently.

[Wladimir Palant, 2013-04-19 13:31:19]

I integrated a bunch of your changes already to avoid further bitrot, commented
on the remaining ones.

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp
File libadblockplus.gyp (right):

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp#newcode3
libadblockplus.gyp:3: 'python%': 'python',
You might want to explain briefly why you need Python to be parametrized
(probably a comment in the build file).

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp#newcode18
libadblockplus.gyp:18: 'msvs_cygwin_shell': 0,
Wouldn't we want that setting for all targets?

[Eric, 2013-04-19 13:55:49]

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp
File libadblockplus.gyp (right):

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp#newcode3
libadblockplus.gyp:3: 'python%': 'python',
On 2013/04/19 13:31:19, Wladimir Palant wrote:
> You might want to explain briefly why you need Python to be parametrized
> (probably a comment in the build file).

Comment added in my working version. It'll be present the next time I offer this
change.

http://codereview.adblockplus.org/10329009/diff/1/libadblockplus.gyp#newcode18
libadblockplus.gyp:18: 'msvs_cygwin_shell': 0,
On 2013/04/19 13:31:19, Wladimir Palant wrote:
> Wouldn't we want that setting for all targets?

We would, in fact, but the msvs generator does not support that specification.
It only looks for this option here; there's no global setting for it.

Similarly for msvs_quote_cmd below. That one is only looked for on the action
itself.

Yeah. I know.