As the library is only built in the master port, and therfore missing in the
build directory for the slave ports, find it via pkg-config and link against it,
when building a slave.

--- libproxy/cmake/pxmodule.cmk.orig	2017-05-11 09:33:33 UTC
+++ libproxy/cmake/pxmodule.cmk
@@ -18,7 +18,12 @@ function(px_module name build builtin)
       set(built "m")
       set_property(SOURCE modules/${name}.cpp PROPERTY COMPILE_DEFINITIONS LIBEXECDIR="${LIBEXECDIR}")
       add_library(${name} MODULE modules/${name}.cpp)
-      target_link_libraries(${name} libproxy)
+      link_directories(${CMAKE_INSTALL_PREFIX}/lib)
+      if (WITH_LIBRARY)
+          target_link_libraries(${name} proxy)
+      else ()
+          target_link_libraries(${name} PkgConfig::LIBPROXY)
+      endif ()
       set_target_properties(${name} PROPERTIES PREFIX "")
       install(TARGETS ${name} LIBRARY DESTINATION ${MODULE_INSTALL_DIR})
       if(${ARGC} GREATER 3)