Please use standard CMake variable to configure static vs. shared builds

[mwoehlke-kitware]

When building jsoncpp as part of a "superbuild" of a larger project, I was surprised that it build a static library. I see this is because shared vs. static is configured by the JSONCPP_LIB_BUILD_SHARED option and not the standard CMake option BUILD_SHARED_LIBS.

Is there a reason you cannot use the latter? Please do so if possible, or at least default JSONCPP_LIB_BUILD_SHARED to ${BUILD_SHARED_LIBS} rather than OFF.

[cdunn2001]

Maybe that was for backward compatibility with scons? But your change to ${BUILD_SHARED_LIBS} seems like a good idea. Could you submit that change?

[cdunn2001]

See #93.