Currently, Gnash's implementation of `ExternalInteface.call()` used a
non-blocking read arrangement when it was trying to read a
return value message from host container, which (in most case, if not always)
caused it to miss the message, and returned `undefined` value to user's
ActionScript code instead of the expected return value.

This patch adds a `select()` call to make `ExternalInterface::readBrowser()`
wait for the message (up to 10 sec); partially fixing bug #46131.

The patch should not cause any other problem, as `ExternalInterface::readBrowser()`
is not used anywhere else in Gnash, and this is essentially what
the old code did, before non-blocking behavior was introduced.

Might need a review since I'm not very familiar with the `select()` function.

Gnash: 0.8.11dev (patched against git f0f66ce 23-Sep-2015) NPAPI
Browser: Iceweasel 10.0.12 (debian)
System: Debian GNU/Linux 7.0 Wheezy i386