[devhelp] build: Use LT_LIB_M

[Sébastien Wilmet <swilmet src gnome org>]

commit ec34bd336753ac387eed1b64f69acbfb105d040e
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Thu Nov 10 16:30:05 2016 -0600

    build: Use LT_LIB_M
    
    LT_LIB_M defines $(LIBM).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=774069

 configure.ac |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 8a170ac..8c40d78 100644
--- a/configure.ac
+++ b/configure.ac
@@ -47,8 +47,7 @@ GOBJECT_INTROSPECTION_CHECK([1.30.0])
 AX_REQUIRE_DEFINED([AX_COMPILER_FLAGS])
 AX_COMPILER_FLAGS
 
-AC_CHECK_LIBM
-AC_SUBST(LIBM)
+LT_LIB_M
 
 AX_REQUIRE_DEFINED([PKG_CHECK_MODULES])