diff --git a/acinclude.m4 b/acinclude.m4
index c10437d2..42ebc789 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -359,8 +359,9 @@ if test "x$acx_pthread_ok" = xyes; then
         AC_MSG_CHECKING([if more special flags are required for pthreads])
         flag=no
         case "${host_cpu}-${host_os}" in
-                *-aix* | *-freebsd*)     flag="-D_THREAD_SAFE";;
-                *solaris* | alpha*-osf*) flag="-D_REENTRANT";;
+                *-aix* | *-freebsd*) flag="-D_THREAD_SAFE";;
+                alpha*-osf*)         flag="-D_REENTRANT";;
+                *solaris*)           flag="-D_REENTRANT -D_POSIX_PTHREAD_SEMANTICS";;
         esac
         AC_MSG_RESULT(${flag})
         if test "x$flag" != xno; then