The D compiler normally predefines several versions that indicate what type of system and machine is in use. They are obviously different on each target supported by GCC.
Version relating to the AArch64 family of processors.
Version relating to the Android platform.
Versions relating to the ARM family of processors.
Version relating to the Cygwin environment.
Versions relating to DragonFlyBSD systems.
Versions relating to FreeBSD systems. The FreeBSD major version number is inferred from the target triplet.
Versions relating to the HPPA family of processors.
Version relating to GNU Hurd systems.
Version relating to Linux systems.
Version relating to the MinGW environment.
Versions relating to the MIPS family of processors.
Version relating to NetBSD systems.
Version relating to OpenBSD systems.
Version relating to OSX systems.
Version relating to POSIX systems (includes Linux, FreeBSD, OSX, Solaris, etc).
Versions relating to the PowerPC family of processors.
Versions relating to the RISC-V family of processors.
Versions relating to the S/390 and System Z family of processors.
Versions relating to Solaris systems.
Versions relating to the SPARC family of processors.
Versions relating to the x86-32 and x86-64 family of processors.
Versions relating to Microsoft Windows systems.