Only build the library in the master port.

--- bindings/CMakeLists.txt.orig	2017-05-11 09:33:33 UTC
+++ bindings/CMakeLists.txt
@@ -20,6 +20,7 @@ if (WITH_PERL)
 endif()
 
 # Install Vala bindings / Install only when -DWITH_VALA is passed to cmake.
+if (WITH_LIBRARY)
 option(WITH_VALA "Enables Vala bindings" OFF)
 if(WITH_VALA)
   if (NOT VAPI_DIR)
@@ -27,4 +28,5 @@ if(WITH_VALA)
   endif()
   install(FILES vala/libproxy-1.0.vapi DESTINATION ${VAPI_DIR})
   set(VAPI_DIR ${VAPI_DIR} CACHE PATH "Vala API directory")
+endif()
 endif()
