Adding Issue #1157 to put back

This is the Pixhawk research board, not the PX4 production board.

Our included make file doesn't build for 64-bit.
We actually need better selection on Windows and Linux platforms for building 32-bit or 64-bit versions.

This causes problems for some systems with Qt5 and Qt4 installed (like Fedora 20).

Added an exists() call beforehand.

I was unsure how to update the QwtPlotPrintFilter code, so I just commented it out with a FIXME.

 (originally removed in 436d0f78)

Settings in user_config.pri are overridden by those specified at the command line when calling qmake.
Also updated the README and user_config.pri.dist to be a little more clear about specifying these other options.

Also fixed some code in QGCPX4VehicleConfig to remove unnecessary DialogBare references.

Manual override through DISABLE_SPEECH now works on all platforms.

It can now be manually disabled.