Use <cmath> instead of <math.h> to fix gcc 6.x build

This patch fixes the following build issue:

../lib/math.h:2577:1: error: ‘int signbit(float)’ conflicts with a previous declaration

that occurs with gcc 6.x.

Patch taken from
https://raw.githubusercontent.com/openembedded/meta-openembedded/master/meta-networking/recipes-connectivity/lftp/files/fix-gcc-6-conflicts-signbit.patch

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

--- lftp-4.6.3a/src/NetAccess.cc.orig	2016-06-18 10:25:51.063358981 -0400
+++ lftp-4.6.3a/src/NetAccess.cc	2016-06-18 10:26:04.143359209 -0400
@@ -21,7 +21,7 @@
 
 #include <errno.h>
 #include <assert.h>
-#include <math.h>
+#include <cmath>
 #include <sys/types.h>
 
 #include "NetAccess.h"
--- lftp-4.6.3a/src/Speedometer.cc.orig	2016-06-18 10:24:58.895358073 -0400
+++ lftp-4.6.3a/src/Speedometer.cc	2016-06-18 10:25:10.879358281 -0400
@@ -18,7 +18,7 @@
  */
 
 #include <config.h>
-#include <math.h>
+#include <cmath>
 #include <stdlib.h>
 #include "Speedometer.h"
 #include "misc.h"
--- lftp-4.6.3a/src/FileCopy.cc.orig	2016-06-18 10:24:15.939357325 -0400
+++ lftp-4.6.3a/src/FileCopy.cc	2016-06-18 10:24:24.583357475 -0400
@@ -36,7 +36,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
-#include <math.h>
+#include <cmath>
 #include <stddef.h>
 #include "FileCopy.h"
 #include "url.h"
--- lftp-4.6.3a/src/ResMgr.cc.orig	2016-06-18 10:23:31.387356549 -0400
+++ lftp-4.6.3a/src/ResMgr.cc	2016-06-18 10:23:41.771356729 -0400
@@ -23,7 +23,7 @@
 #include <ctype.h>
 #include <unistd.h>
 #include <stdlib.h>
-#include <math.h>
+#include <cmath>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <errno.h>