[CMake] FindJsoncpp.cmake: search pkg-config libs in default search paths.

pkg_search_module(JSONCPP) should set
JSONCPP_LIBDIR/JSONCPP_LIBRARY_DIRS to where the libjsoncpp.so can be
found. However, on Ubuntu 14.04 LTS (Trusty Tahr) it returns /usr/lib
while the libjsoncpp library can be found at
/usr/lib/x86_64-linux-gnu/libjsoncpp.so. Thus, while searching for
the full path of the jsoncpp library, it is not found.

JSONCPP_LIBDIR is correctly set to /usr/lib/x86_64-linux-gnu on e.g.,
Ubuntu 16.04 LTS (Xenial Xerus )

Fix by removing the NO_DEFAULT_PATH flag, in order to search the system
default paths even if the library is not found in
JSONCPP_LIBDIR/JSONCPP_LIBRARY_DIRS.

This fixes bug llvm.org/PR33798.

llvm-svn: 308287