bpo-38301: In Solaris family, we must be sure to use '-D_REENTRANT' (#16446)

Misc/NEWS.d/next/Build/2019-09-28-02-37-11.bpo-38301.123456.rst

+In Solaris family, we must be sure to use ``-D_REENTRANT``.
+Patch by Jesús Cea Avión.

configure

@@ -10500,6 +10500,9 @@ then
     $as_echo "#define _REENTRANT 1" >>confdefs.h
 
     posix_threads=yes
+    if test "$ac_sys_system" = "SunOS"; then
+        CFLAGS="$CFLAGS -D_REENTRANT"
+    fi
 elif test "$ac_cv_kpthread" = "yes"
 then
     CC="$CC -Kpthread"

configure.ac

@@ -3063,6 +3063,9 @@ then
     # Defining _REENTRANT on system with POSIX threads should not hurt.
     AC_DEFINE(_REENTRANT)
     posix_threads=yes
+    if test "$ac_sys_system" = "SunOS"; then
+        CFLAGS="$CFLAGS -D_REENTRANT"
+    fi
 elif test "$ac_cv_kpthread" = "yes"
 then
     CC="$CC -Kpthread"