class library: jitlib: make sure GUI.skins is not nil in ProxyMixerOld
cmake build system: link with boost.thread
HelpSource: MIDIFunc: show how to free a MIDIFunc

When using several MIDIFunc, assigning all of them to the same variable
will lead to confusing results as each MIDIFunc will continue to exist
on top of the old one.

This commit shows the good practice: add a variable to each MIDIFunc
example line and show how to free them.
Merge pull request #893 from redFrik/master

make sure GUI.skins is not nil in ProxyMixerOld
Merge pull request #900 from gusano/topic/MIDIFunc_helpSource_improvement

HelpSource: MIDIFunc: show how to free a MIDIFunc
class library: fix curvelin

fixes #902

Signed-off-by: Tim Blechmann <tim@klingt.org>
class library: UGen - avoid numeric instability in lincurve/curvelin

Signed-off-by: Tim Blechmann <tim@klingt.org>
class library: ugen - use Select.multiNew

Signed-off-by: Tim Blechmann <tim@klingt.org>