[gnome-directory-thumbnailer] build: Use AC_CHECK_LIBM to check for libm

[Philip Withnall <pwithnall src gnome org>]

commit f6ce2d160f77d8964379c5458aac3461dcf9911b
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Tue Jun 9 13:31:51 2015 +0100

    build: Use AC_CHECK_LIBM to check for libm

 Makefile.am  |    2 +-
 configure.ac |    3 +++
 2 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/Makefile.am b/Makefile.am
index 27a886a..78ee2eb 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -33,7 +33,7 @@ src_gnome_directory_thumbnailer_CFLAGS = \
 
 src_gnome_directory_thumbnailer_LDADD = \
        $(GDT_LIBS) \
-       -lm \
+       $(LIBM) \
        $(CODE_COVERAGE_LIBS) \
        $(AM_LIBADD) \
        $(NULL)
diff --git a/configure.ac b/configure.ac
index 3a460bb..b2b0773 100644
--- a/configure.ac
+++ b/configure.ac
@@ -39,6 +39,9 @@ AC_SUBST([GDT_VERSION_MICRO])
 # gnome-directory-thumbnailer dependencies
 PKG_CHECK_MODULES(GDT, [gobject-2.0 glib-2.0 >= $GLIB_REQS gio-2.0 >= $GIO_REQS gdk-pixbuf-2.0 >= 
$GDK_PIXBUF_REQS gtk+-3.0 >= $GTK_REQS gnome-desktop-3.0 >= $GNOME_DESKTOP_REQS])
 
+AC_CHECK_LIBM
+AC_SUBST([LIBM])
+
 # Internationalisation
 GETTEXT_PACKAGE=AC_PACKAGE_NAME
 AC_SUBST([GETTEXT_PACKAGE])