configure script ensures use of -lm

configure.ac

@@ -81,6 +81,9 @@ case $target in
         ;;
 esac
 
+AC_CHECK_LIBM
+AC_SUBST(LIBM)
+
 # optionally choose the MQ plugins to build, by default all
 AC_ARG_ENABLE([kafka], [AS_HELP_STRING([--enable-kafka],
                        [build the Kafka MQ plugin @<:@default=yes@:>@])],

src/Makefile.am

@@ -25,7 +25,7 @@ trackrdrd_SOURCES = \
 	vtim.c
 
 trackrdrd_LDADD = \
-	${PTHREAD_LIBS} @VARNISH_LIBS@ \
+	${PTHREAD_LIBS} @VARNISH_LIBS@ ${LIBM} \
 	-L${VARNISH_PKG_LIB}/varnish -lvarnish
 
 trackrdrd_LDFLAGS = -ldl
@@ -85,4 +85,4 @@ vcs_version.h: FORCE
 		fi \
 	fi
 
-FORCE:
\ No newline at end of file
+FORCE: