Similar to the existing macros for compile options and link options,
this macro wraps the command line definitions for swiftc in a way that
avoids warnings about conditional compilation flags not having values.
Skia now uses GPU-accelerated painting on Linux if Vulkan is available.
Most of the performance gain is currently negated by reading the GPU
backend back into RAM to pass it to the Browser process. In the future,
this could be improved by sharing GPU-allocated memory across the
Browser and WebContent processes.
