libgo: check for makecontext in -lucontext

Patch from Sören Tempel.

Change-Id: I74b4052cc7aca19224afd6d9a2739c3fae46b722
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/458396
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

libgo/configure

@@ -14818,6 +14818,63 @@ fi
 
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing makecontext" >&5
+$as_echo_n "checking for library containing makecontext... " >&6; }
+if ${ac_cv_search_makecontext+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_func_search_save_LIBS=$LIBS
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char makecontext ();
+int
+main ()
+{
+return makecontext ();
+  ;
+  return 0;
+}
+_ACEOF
+for ac_lib in '' ucontext; do
+  if test -z "$ac_lib"; then
+    ac_res="none required"
+  else
+    ac_res=-l$ac_lib
+    LIBS="-l$ac_lib  $ac_func_search_save_LIBS"
+  fi
+  if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_search_makecontext=$ac_res
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext
+  if ${ac_cv_search_makecontext+:} false; then :
+  break
+fi
+done
+if ${ac_cv_search_makecontext+:} false; then :
+
+else
+  ac_cv_search_makecontext=no
+fi
+rm conftest.$ac_ext
+LIBS=$ac_func_search_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_search_makecontext" >&5
+$as_echo "$ac_cv_search_makecontext" >&6; }
+ac_res=$ac_cv_search_makecontext
+if test "$ac_res" != no; then :
+  test "$ac_res" = "none required" || LIBS="$ac_res $LIBS"
+
+fi
+
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for library containing sched_yield" >&5
 $as_echo_n "checking for library containing sched_yield... " >&6; }
 if ${ac_cv_search_sched_yield+:} false; then :

libgo/configure.ac

@@ -570,6 +570,9 @@ PTHREAD_LIBS=
 AC_CHECK_LIB([pthread], [pthread_create], PTHREAD_LIBS=-lpthread)
 AC_SUBST(PTHREAD_LIBS)
 
+dnl Test if -lucontext is required for makecontext.
+AC_SEARCH_LIBS([makecontext], [ucontext])
+
 dnl Test if -lrt is required for sched_yield or nanosleep or clock_gettime.
 AC_SEARCH_LIBS([sched_yield], [rt])
 AC_SEARCH_LIBS([nanosleep], [rt])