Properly detect when to use -lm

http://sf.net/tracker/?func=detail&aid=2779880&group_id=22615&atid=375631

configure.in

@@ -10,6 +10,8 @@ dnl AC_PROG_YACC
 AC_PROG_LEX
 
 AC_CHECK_LIB(resolv, inet_aton)
+AC_CHECK_LIBM
+AC_SUBST(LIBM)
 
 AC_PATH_PROGS([M4], [gm4 gnum4 m4], [notfound],[/usr/local/bin:/usr/freeware/bin:/usr/gnu/bin:${prefix}/bin:${exec_prefix}/bin:$PATH] )
 if test "x$M4" = "xnotfound"; then

src/Makefile.am

@@ -1,12 +1,11 @@
 bin_PROGRAMS = docsis
 docsis_SOURCES = docsis.c ethermac.c md5.c docsis_encode.c docsis_decode.c docsis_snmp.c docsis_yy.y docsis_lex.l hmac_md5.c
-docsis_LDADD = -lm
 
 # set the include path found by configure
 INCLUDES = $(all_includes) $(NETSNMP_CFLAGS)
 
 # the library search path.
-docsis_LDFLAGS = $(all_libraries) $(NETSNMP_LIBS) $(LEXLIB)
+docsis_LDFLAGS = $(all_libraries) $(NETSNMP_LIBS) $(LEXLIB) $(LIBM)
 noinst_HEADERS = ethermac.h md5.h docsis.h docsis_common.h docsis_decode.h docsis_encode.h docsis_symtable.h docsis_snmp.h docsis_globals.h
 EXTRA_DIST=docsis_yy.h
 CLEANFILES=docsis_yy.c docsis_yy.h docsis_yy.output docsis_lex.c