Reverse conditional compilation of non-POSIX

As https://www.openbsd.org/faq/ports/guide.html#PortsGeneric states, it
is inappropriate to base feature inclusion on a growing list (60b66fc,
9160d12) of excluded platforms. Thus this commit attempts to switch the
assumption from Linux to a modern POSIX as the default target.

Firstly, merely checking for __unix__ should be functionally equivalent
of having a long lists of practically "all" operating systems.

Apart from Windows, the primary non-standard code covers these termios
related constants: I_IUCLC, O_OLCUC, O_OFILL, O_OFDEL, O_NLDLY, O_CRDLY
and L_XCASE.

A few of them are documented as LEGACY as they were removed from POSIX
https://pubs.opengroup.org/onlinepubs/007904875/basedefs/termios.h.html
over two decades ago. The other constants are part of the X/Open System
Interfaces, which supposedly should be available when _XOPEN_UNIX is set
to another value than -1. However on Linux they exist, yet the variable
indicating their presence is undefined.

In summary;   It is hard to find any feature specific preprocessor
variables indicating availability, but it is quite safe to limit their
inclusion to Linux until patches arrive for other platforms.

Executing `cpp -dM </dev/null | grep -i linux` on contemporary glibc and
musl based distributions tend to give a few constants, where __linux__
likely is the one most suitable.

Based on the previous state of the code, the following constants are
expected to be available also on Apple: O_BSDLY, O_VTDLY and O_FFDLY.
They do not appear be documented in termios(4) on macOS 13, but they
actually exist in /Library/Developer/…/sys/termios.h.

Lastly, O_TABDLY is kept excluded exclusively for NetBSD as that is the
only currently supported platform lacking it.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/random.c b/src/random.c
index 70bf13b..7bffe30 100644
--- a/src/random.c
+++ b/src/random.c
@@ -18,7 +18,7 @@
     #include <string.h>
     #if defined(__linux__)
         #include <sys/random.h> // getrandom()
-    #elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
+    #elif defined(__APPLE__) || defined(__unix__)
         #include <stdlib.h>     // arc4random_buf()
     #endif
 #endif
@@ -80,7 +80,7 @@ static int lua_get_random_bytes(lua_State* L) {
         total_read += n;
     }
 
-#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
+#elif defined(__APPLE__) || defined(__unix__)
     // Use arc4random_buf() on BSD/macOS
     arc4random_buf(buffer, num_bytes);