Add option JSONCPP_WITH_EXAMPLE (open-source-parsers#1099)

* Add option JSONCPP_WITH_EXAMPLE

Allows to conditionally build examples as
it has been done for tests.  Useful for packaging.

* Do not build example by default

CMakeLists.txt

@@ -82,6 +82,7 @@ option(JSONCPP_WITH_WARNING_AS_ERROR "Force compilation to fail if a warning occ
 option(JSONCPP_WITH_STRICT_ISO "Issue all the warnings demanded by strict ISO C and ISO C++" ON)
 option(JSONCPP_WITH_PKGCONFIG_SUPPORT "Generate and install .pc files" ON)
 option(JSONCPP_WITH_CMAKE_PACKAGE "Generate and install cmake package files" ON)
+option(JSONCPP_WITH_EXAMPLE "Compile JsonCpp example" OFF)
 option(BUILD_SHARED_LIBS "Build jsoncpp_lib as a shared library." OFF)
 
 # Enable runtime search path support for dynamic libraries on OSX
@@ -228,4 +229,6 @@ add_subdirectory( src )
 add_subdirectory( include )
 
 #install the example
-add_subdirectory( example )
+if(JSONCPP_WITH_EXAMPLE)
+  add_subdirectory( example )
+endif()