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.
Workaround for #51.

Author: akien-mga

src/lib_json/CMakeLists.txt

@@ -1,4 +1,8 @@
 OPTION(JSONCPP_LIB_BUILD_SHARED "Build jsoncpp_lib as a shared library." OFF)
+IF(BUILD_SHARED_LIBS)
+  SET(JSONCPP_LIB_BUILD_SHARED ON)
+ENDIF(BUILD_SHARED_LIBS)
+
 IF(JSONCPP_LIB_BUILD_SHARED)
   SET(JSONCPP_LIB_TYPE SHARED)
   ADD_DEFINITIONS( -DJSON_DLL_BUILD )