When targeting darwin, clang requires commas between arguments,
while the no-comma form is allowed for other targets.

Since Xcode 9.3, the bundled clang supports altmacro and doesn't
require using gas-preprocessor any longer.
Signed-off-by: Martin Storsjö <martin@martin.st>

uses comma as macro parameter separator
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>