bpo-35550: Fix incorrect Solaris define guards (GH-11275)


Python source code uses on several places ifdef sun or defined(sun) without the underscores, which is not standard compliant and shouldn't be used.

Defines should check for __sun instead. Reference: http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_systemGH-Solaris

https://bugs.python.org/issue35550
(cherry picked from commit 6f9bc72c79c3262e5d0f2c0e96b016477399cfb1)

Co-authored-by: Jakub KulĂ­k <Kulikjak@gmail.com>
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 383999e..909b06e 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -5926,7 +5926,7 @@
 #endif
 #if defined(HAVE_DEV_PTMX) && !defined(HAVE_OPENPTY) && !defined(HAVE__GETPTY)
     PyOS_sighandler_t sig_saved;
-#ifdef sun
+#if defined(__sun) && defined(__SVR4)
     extern char *ptsname(int fildes);
 #endif
 #endif