https://bugs.gentoo.org/953896

pty.h is necessary on some Linux, but it isn't in POSIX, so it breaks
systems like macOS, *BSD, HPUX, AIX and Solaris.  Basically everywhere
not-Linux, so add guard to only apply on Linux.

--- a/pty.c
+++ b/pty.c
@@ -29,6 +29,9 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#if defined(__linux__) || defined(__linux) || defined(linux)
+#include <pty.h>
+#endif
 #include <signal.h>
 
 #if defined(__OpenBSD__)