configure / util: Auto-detect the availability of openpty()

Recent versions of Solaris (v11.4) now feature an openpty() function,
too, causing a build failure since we ship our own implementation of
openpty() for Solaris in util/qemu-openpty.c so far. Since there are
now both variants available in the wild, with and without this function
(and illumos is said to not have this function yet), let's introduce a
proper HAVE_OPENPTY define for this to fix the build failure.

Message-Id: <20200702143955.678-1-thuth@redhat.com>
Tested-by: Michele Denber <denber@mindspring.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -5134,10 +5134,14 @@ extern int openpty(int *am, int *as, char *name, void *termp, void *winp);
 int main(void) { return openpty(0, 0, 0, 0, 0); }
 EOF
 
-if ! compile_prog "" "" ; then
+have_openpty="no"
+if compile_prog "" "" ; then
+  have_openpty="yes"
+else
   if compile_prog "" "-lutil" ; then
     libs_softmmu="-lutil $libs_softmmu"
     libs_tools="-lutil $libs_tools"
+    have_openpty="yes"
   fi
 fi
 
@@ -7380,6 +7384,9 @@ fi
 if test "$have_broken_size_max" = "yes" ; then
     echo "HAVE_BROKEN_SIZE_MAX=y" >> $config_host_mak
 fi
+if test "$have_openpty" = "yes" ; then
+    echo "HAVE_OPENPTY=y" >> $config_host_mak
+fi
 
 # Work around a system header bug with some kernel/XFS header
 # versions where they both try to define 'struct fsxattr':

util/qemu-openpty.c

@@ -52,7 +52,9 @@
 #endif
 
 #ifdef __sun__
-/* Once Solaris has openpty(), this is going to be removed. */
+
+#if !defined(HAVE_OPENPTY)
+/* Once illumos has openpty(), this is going to be removed. */
 static int openpty(int *amaster, int *aslave, char *name,
                    struct termios *termp, struct winsize *winp)
 {
@@ -93,6 +95,7 @@ err:
         close(mfd);
         return -1;
 }
+#endif
 
 static void cfmakeraw (struct termios *termios_p)
 {