math: Ensure declarations of math functions.

[gnulib.git] / m4 / cbrtl.m4

diff --git a/m4/cbrtl.m4 b/m4/cbrtl.m4
index f40a1c0..b2a1a7c 100644 (file)
--- a/m4/cbrtl.m4
+++ b/m4/cbrtl.m4
@@ -1,4 +1,4 @@
-# cbrtl.m4 serial 2
+# cbrtl.m4 serial 3
 dnl Copyright (C) 2012 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -11,6 +11,9 @@ AC_DEFUN([gl_FUNC_CBRTL],
   AC_REQUIRE([gl_LONG_DOUBLE_VS_DOUBLE])
   AC_REQUIRE([gl_FUNC_CBRT])
 
+  dnl Persuade glibc <math.h> to declare cbrtl().
+  AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+
   dnl Test whether cbrtl() exists. Assume that cbrtl(), if it exists, is
   dnl defined in the same library as cbrt().
   save_LIBS="$LIBS"