Check for libhdfs configure-time.

[msiebuhr]

Instead of failing on missing libhdfs build-time, it now complains at configure-time.

[msiebuhr]

I'm cleaning up old repos, and as nothing seem to happen here, I'll delete my repo and dump the diff here, just in case:

diff --git a/wscript b/wscript
index f48ff33..506b4fd 100644
--- a/wscript
+++ b/wscript
@@ -9,6 +9,7 @@ def set_options(opt):
 def configure(conf):
   conf.check_tool("compiler_cxx")
   conf.check_tool("node_addon")
+  conf.check_cxx(lib='hdfs', mandatory=True)

 def build(bld):
   obj = bld.new_task_gen("cxx", "shlib", "node_addon", includes='./src ./vendor', linkflags=['-lhdfs'])