Configure: Moved check for Math lib to a dedicated M4 macro

Took the opportunity to rewrite the check using autoconf macros
to generate a more compatible configure script.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

configure.ac

@@ -477,12 +477,7 @@ dnl
 dnl libWINGS uses math functions, check whether usage requires linking
 dnl against libm
 dnl
-AC_CHECK_FUNC(atan,[mathneedslibm=no;LIBM=],[mathneedslibm=dunno])
-if test "x$mathneedslibm" = "xdunno" ; then
-	AC_CHECK_LIB(m, atan, [LIBM=-lm])
-fi
-AC_SUBST(LIBM)
-
+WM_CHECK_LIBM
 
 dnl
 dnl libWINGS uses FcPatternDel from libfontconfig

m4/wm_libmath.m4

@@ -0,0 +1,32 @@
+# wm_libmath.m4 - Macros to check proper libMath usage for WINGs
+#
+# Copyright (c) 2013 Christophe Curis
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+# WM_CHECK_LIBM
+# -------------
+#
+# Checks the needed library link flags needed to have math lib
+# Sets variable LIBM with the appropriates flags
+AC_DEFUN_ONCE([WM_CHECK_LIBM],
+[AC_CHECK_FUNC(atan,
+    [LIBM=],
+    [AC_CHECK_LIB(m, [atan],
+        [LIBM=-lm],
+        [AC_MSG_WARN(Could not find Math library, you may experience problems)
+         LIBM=] )] ) dnl
+AC_SUBST(LIBM) dnl
+])