[source] iperf: add -lm to fix build with newer glibc

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/98206cb9c62a86f3e72b74a71e9ed836bac1fc71

commit 98206cb9c62a86f3e72b74a71e9ed836bac1fc71
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Mon Aug 29 15:09:50 2016 +0200

    iperf: add -lm to fix build with newer glibc
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 package/network/utils/iperf/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/package/network/utils/iperf/Makefile b/package/network/utils/iperf/Makefile
index 2bd1917..c47830c 100644
--- a/package/network/utils/iperf/Makefile
+++ b/package/network/utils/iperf/Makefile
@@ -41,7 +41,7 @@ TARGET_CFLAGS += -D_GNU_SOURCE
 CONFIGURE_ARGS += --disable-multicast
 
 CONFIGURE_VARS += CXXFLAGS="$$$$CXXFLAGS -fno-rtti"
-CONFIGURE_VARS += LIBS="-lpthread"
+CONFIGURE_VARS += LIBS="-lpthread -lm"
 
 define Package/iperf/install
 	$(INSTALL_DIR) $(1)/usr/bin