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.

Can now be disabled by the DISABLE_KINECT define.
Also better output for users when running qmake.

Added documentation and clarified messages to the user.

This feature can be enabled at compile-time and documentation has been added to the README.

XBee support can now be disabled by defining DISABLE_XBEE.
Details on why XBee support is disabled are now output.

Speech synthesis support can now be disabled (DEFINES+=DISABLE_SPEECH)
Speech synthesis dependencies are now checked before enabling.
When support cannot be enabled, the user is now notified.

MAVLink dialects can now be specified as a list, so multiple dialects can be compiled in simultaneously (may result in conflicts between dialects).
MAVLink dialects are checked for existance before support is added.
Users are notified of which dialects are used and which cannot be added and why.
The README now has info about specifying MAVLink dialects.

MAVLink dialects can now be specified as a list, so multiple dialects can be compiled in simultaneously (may result in conflicts between dialects).
MAVLink dialects are checked for existance before support is added.
Users are notified of which dialects are used and which cannot be added and why.
The README now has info about specifying MAVLink dialects.

Now properly checks for if QUpgrade submodule has been cloned.
Allows for disabled QUpgrade compilation.
Altered some code so that compiling without the QUpgrade module could
succeed.
Updated README about QUpgrade submodule.

Conflicts:
	QGCExternalLibs.pri
	src/GAudioOutput.cc

Causing my TeamCity builds to fail

Made them more understandable for what is needed and for why. Removed
duplication and made sure builds used common settings across all OS
flavors. Removed mingw support for Windows since it was only partially
implemented.