When building zst under cygwin or msys2 with std=c99 the build would fail because
of an undefined fileno()/_fileno(), which is used by the IS_CONSOLE() macro.
When building with -std=c99 (gcc otherwise defaults to gnu, which implies POSIX),
which is the default of the cmake build, then including unistd.h wont define
_POSIX_VERSION and all other headers also wont expose POSIX API.
To fix this make sure to define _POSIX_C_SOURCE with the version we want before including
unistd.h and so that _POSIX_VERSION is set to the version provided by the system.
Since Cygwin/MSYS2 just follow POSIX we can also remove their special cases for
defining IS_CONSOLE().
And, for completeness, also explicitly include stdio.h which is what actually declares fileno().
Tested with the normal make file and cmake under MSYS2 and Cygwin.
Cygwin currently uses the fallback implementation which just returns 1 every time,
which leads to bad performance when zstd is called with -T0 for example.
Instead use the POSIX implementation used for the BSDs which works just fine under Cygwin.
Tested under Cygwin and MSYS2.
Christoph Reiter