depend on fontconfig when building for linux rather than x11

depend on fontconfig for linux rather than x11

https://codereview.chromium.org/99103006 moved WebFontInfo.cpp from
core to platform.  In core fontconfig is enabled if OS=="linux",
but in platform it is enabled if use_x11==1.  In most setups this
works because the headers are in the same base directory, but if
that's not the case the build will fail.

To fix this, I think platform should just depend on fontconfig if
OS=="linux".

BUG=303332
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=164428

[Mostyn Bramley-Moore, 2014-01-02 11:12:47]

Spang, rjkroege, Stephen: does this change look OK?

[Stephen Chennney, 2014-01-02 15:21:47]

lgtm

Good catch. LGTM.

[commit-bot: I haz the power, 2014-01-02 15:21:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mostynb@opera.com/122653004/1

[commit-bot: I haz the power, 2014-01-02 19:45:18]

Retried try job too often on win_layout for step(s) webkit_lint
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_layout...

[commit-bot: I haz the power, 2014-01-02 21:14:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mostynb@opera.com/122653004/1

[commit-bot: I haz the power, 2014-01-02 21:14:43]

Change committed as 164428