Fixing memory leak in cocoa_joystick

Making sure that propsRef and valueRef get released no matter when we
break out of the loop.

Resolves #231.
Added _GLFW_USE_RETINA.
Fixed X11 drop support.

This was broken by 8ae063bb1b61cf0daa9c4d4c10d6a189e8cad121.

Fixes #224.
Updated changelog to include #231.
Allowed drops from less privileged processes.

By default, UAC will prevent less privileged processes from sending
these messages.  This resolves that on Windows 7 and later.  Windows
Vista is still affected by this issue, as it has UAC but lacks
ChangeWindowMessageFilterEx.

Fixes #227.
Enabled pkg-config file generation on MinGW.

Fixes #220.
Blurb nomenclature fix.
Fixed copypaste mistake.
WINVER fixes for MinGW and VC++.
Fixed return value for WM_DROPFILES.
Fixed uncaught BadWindow in EWMH check.

Fixes #234.
Merge remote-tracking branch 'upstream/master'