vcl/osx: restore some pre-10.8 conditional code

related commits:
    ec147012340219c8bb5eaf8fe3d41be7836bc261
    16996c6b91b53e3c7d316296833bdb8a9cb60c22

Change-Id: Ie8962f8d26f163eed91fdf6b137b0d9830183dd2
Use GNU objc ABI/runtime instead of NeXT ABI/runtime when compiler is GCC

also,
  + fix "cannot find interface declaration for NXConstantString"
  + include <objc/objc-api.h> when OBJC_EXPORT is not defined

Change-Id: I5f0df322435be8244d372642dee1c21d783306f1
vcl/osx/salframe.cxx: move includes, objc_msgSend workaround

Change-Id: I39886b4590f227ec69042fed0f5240ba0b0fd7f2
Don't assume that /usr/bin/mktemp is always a BSD mktemp on OSX, add backup for GNU mktemp

Change-Id: Ifb119dd13062593cba5aa3cb1dacc54194339544