gh-129141: Fix build on gcc-9.4.0

[mdboom]

d3b1bb2 introduced the line:

# if USING_APPLE_OS_LIBFFI && defined(__has_builtin) && __has_builtin(__builtin_available) 

Unfortunately this doesn't work on compilers that don't have __has_builtin, because the parser fails on the usage of __has_builtin(__builtin_available) before is has a change to run the test defined(__has_builtin). Separating the defined check to its own expression seems to resolve this.

• Issue: Build fails with gcc 9.4.0 after d3b1bb2 #129141

[picnixz]

Mmh. Build bots don't seem to be running.

[encukou]

Thank you!
I don't see the failure on main buildbots, so I don't think PR buildbots will bring us anything here. I'll trust your local testing.