Make CAPNP_EXECUTABLE and friend user-overrideable

These variables need to be cache variables or else the user can't override them on the command line (e.g. when building a project against a lite mode installation).

c++/cmake/CapnProtoConfig.cmake.in

@@ -38,8 +38,10 @@
 
 set(CapnProto_VERSION @VERSION@)
 
-set(CAPNP_EXECUTABLE $<TARGET_FILE:CapnProto::capnp_tool>)
-set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:CapnProto::capnpc_cpp>)
+set(CAPNP_EXECUTABLE $<TARGET_FILE:CapnProto::capnp_tool>
+    CACHE FILEPATH "Location of capnp executable")
+set(CAPNPC_CXX_EXECUTABLE $<TARGET_FILE:CapnProto::capnpc_cpp>
+    CACHE FILEPATH "Location of capnpc-c++ executable")
 set(CAPNP_INCLUDE_DIRECTORY "@PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR@")
 
 # work around http://public.kitware.com/Bug/view.php?id=15258