Require c99 when building libhdfs

Per the discussion in HDFS-1619, libhdfs uses some c99 features, therefore we should use compile with c99 standard flags (eg c99 or gnu99 when using gcc) when building it. We could do this with autotools via AC_PROG_CC_C99 (which requires a more recent autotools) or by setting CLFAGS in src/c+/libhdfs/configure. We should perhaps rename src/c+ to src/c while we're at it since libhdfs is not written in c++ and libhdfs is the only subdirectory.