Use standard CMake variables - static/shared lib.

Currently JSONCPP_LIB_BUILD_SHARED variable is used as option to build static/shared libraries.
The current patch uses standard CMake variables for this.
Such a workaround is done in open-source-parsers#51
Current patch will make it generic.

CMakeLists.txt

@@ -9,6 +9,8 @@ OPTION(JSONCPP_WITH_POST_BUILD_UNITTEST "Automatically run unit-tests as a post
 OPTION(JSONCPP_WITH_WARNING_AS_ERROR "Force compilation to fail if a warning occurs" OFF)
 OPTION(JSONCPP_WITH_PKGCONFIG_SUPPORT "Generate and install .pc files" ON)
 OPTION(JSONCPP_WITH_CMAKE_PACKAGE "Generate and install cmake package files" OFF)
+OPTION(BUILD_SHARED_LIBS "Build jsoncpp_lib as a shared library." OFF)
+OPTION(BUILD_STATIC_LIBS "Build jsoncpp_lib static library." ON)
 
 # Ensures that CMAKE_BUILD_TYPE is visible in cmake-gui on Unix
 IF(NOT WIN32)