[CMake] FindJsoncpp.cmake: Use descriptive variable name for libjsoncpp.so path.

find_library(lib) stores the result in the variable "lib", which is also
cached in CMakeCache.txt.  This could theoretically conflict if jsoncpp
required two libraries, which each would get cached as "lib".  Use a
more descriptive and disambiguative "jsoncpp_${libname}" for that.

llvm-svn: 308289