Adjust the check method of HAVE_PROTOBUF
Adjust the check method of HAVE_PROTOBUF

* On CentOS, for "pkg-config --cflags protobuf" command,  pkg-config 2.9.2
  will get empty string, in stead pkg-config 2.5 will get " ". So
  PROTOBUF_CFLAGS can't be used to check presence of protobuf.

Change-Id: I543a4b353f35f47c666f97132b8108efd2625429
Signed-off-by: Jason Tang <7bffd3ac92c799ededc488fc8ffb5bbc75b0893d@emc.com>
Reviewed-on: https://gerrit.mozy.lab.emc.com/113480
Reviewed-by: Kevin Cai <1855c94a87a57fea8c6ab3706ebae134b80255c2@emc.com>
Reviewed-by: Zhaoding Wu <61420d7370e20bdc7547c2b81077a94cae0ed7e1@emc.com>

mozy/mordor,mozy/mordor,mozy/mordor