Add option JSONCPP_WITH_EXAMPLE (#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
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8a7d3ef..4e06331 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -82,6 +82,7 @@
 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( include )
 
 #install the example
-add_subdirectory( example )
+if(JSONCPP_WITH_EXAMPLE)
+  add_subdirectory( example )
+endif()