compiler.pri: update to Boost 1.54.0 on OS X.
compiler.pri: use short-form OS X SDKs for Qt 5.
mumble: fix VoiceRecorderDialog build on Apple clang-500.1.70.
build: allow Mumble on Win32 to be built against a static Qt.

Also build-tested on OS X.

We currently disable the manual plugin when doing static builds
on Win32, because of manual.dll's Qt dependency.
murmur: build fixes for win32-static.
Fix path in skin selection dialog

* Open skin selection dialog with sane path
** if empty
*** installation dir on windows
*** data dir on linux
*** in skins subfolder, if it exists
** Use currently set path if set
Open soundfile selection dialog with current path