use_dll: On macOS, enforce GCC with mkoctfile

Otherwise Clang will be used, and it does not accept our custom optimization
flags.

Ref. dynare#1797
6 jobs for master in 3 minutes and 42 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_aarch64 #33530

00:01:55

passed build_linux_i686 #33528

00:01:50

passed build_linux_x86_64 #33529

00:01:19

passed build_macos_x86_64 #33533
macOS

00:03:42

passed build_windows_i686 #33531

00:01:30

passed build_windows_x86_64 #33532

00:00:56