BUG 2478: Search for pkgconfig file in /lib or /lib64.

json-c-devel package for CentOS 6.6 (x86_64) puts the pkgconfig file
in /lib64 directory.

Change-Id: If7455a85e79dfaab9a21a65e0186e11e5b5b5db3
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>