[2.7] bpo-38301: In Solaris family, we must be sure to use '-D_REENTRANT' (GH-16446). (#16454)

(cherry picked from commit 52d1b86b)
Co-authored-by: Jesús Cea <jcea@jcea.es>

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

@@ -9602,6 +9602,9 @@ then
 
     posix_threads=yes
     THREADOBJ="Python/thread.o"
+    if test "$ac_sys_system" = "SunOS"; then
+        CFLAGS="$CFLAGS -D_REENTRANT"
+    fi
 elif test "$ac_cv_kpthread" = "yes"
 then
     CC="$CC -Kpthread"

configure.ac

@@ -2625,6 +2625,9 @@ then
     AC_DEFINE(_REENTRANT)
     posix_threads=yes
     THREADOBJ="Python/thread.o"    
+    if test "$ac_sys_system" = "SunOS"; then
+        CFLAGS="$CFLAGS -D_REENTRANT"
+    fi
 elif test "$ac_cv_kpthread" = "yes"
 then
     CC="$CC -Kpthread"