When I build Flowplayer, I stumbled across error messages like:

[exec] Loading configuration file /usr/local/flex_sdk_3.4/frameworks/flex-config.xml
[exec] /home/jj/Work/247fps/git/projector/flowplayer.controls/src/actionscript/org/flowplayer/controls/button/SkinClasses.as(21): col: 32 Error: Type was not found or was not a compile-time constant: FullScreenOnButton.
[exec] 
[exec]             private var foo:fp.FullScreenOnButton;
[exec]                                ^
[exec] 
[exec] /home/jj/Work/247fps/git/projector/flowplayer.controls/src/actionscript/org/flowplayer/controls/button/SkinClasses.as(22): col: 32 Error: Type was not found or was not a compile-time constant: FullScreenOffButton.
[exec] 
[exec]             private var bar:fp.FullScreenOffButton;
[exec]                                ^
[exec] 
[exec] /home/jj/Work/247fps/git/projector/flowplayer.controls/src/actionscript/org/flowplayer/controls/button/SkinClasses.as(23): col: 33 Error: Type was not found or was not a compile-time constant: NextButton.
[exec] 
[exec]             private var next:fp.NextButton;
[exec]
I don't have my own version of controls or anything like that. I finally figured out that in my build.properties file, I have:

plugin.buildfiles=flowplayer.controls/build.xml,flowplayer.controls/build-tube.xml,flowplayer.controls/build-skinless.xml
plugin.buildfiles isn't mentioned eitherhttp://flowplayer.org/documentation/developer/development-environment.html orhttp://flowplayer.org/documentation/developer/writing-flash-plugins.html. However, the tutorial suggests you have things like flowplayer.controls at the same level as flowplayer.core.

Setting the following fixed my problem:

plugin.buildfiles=../flowplayer.controls/build.xml, 
                  ../flowplayer.controls/build-tube.xml, 
		  ../flowplayer.controls/build-skinless.xml
It might make sense to document this on one of those two pages.

Hopefully that helps other people out there who stumble across this same problem :)