explicitly link against -lm

Author: bwalex

ext/rbczmq/extconf.rb

@@ -158,6 +158,7 @@ def check_heads heads = [], fatal = false
 fail "Error compiling and linking libsodium" unless have_library("sodium")
 fail "Error compiling and linking libzmq" unless have_library("zmq")
 fail "Error compiling and linking libczmq" unless have_library("czmq")
+fail "Error linking against libm" unless have_library("m")
 
 $defs << "-pedantic"