Adapt libdir for 64bit RPM-based distros

RPM-based distros such as Fedora or Mageia put 64bit libraries in /usr/lib64
while 32bit libraries go to /usr/lib. This is usually taken into account
in CMake projects using a LIB_SUFFIX parameter that can be set to "" or "64".
Add support for BUILD_SHARED_LIBS argument

BUILD_SHARED_LIBS is a standard CMake argument that serves the purpose
of the custom JSONCPP_LIB_BUILD_SHARED. For now we force JSONCPP_LIB_BUILD_SHARED
to true if BUILD_SHARED_LIBS was defined.