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.

--- utils/CMakeLists.txt.orig	2017-05-11 09:33:33 UTC
+++ utils/CMakeLists.txt
@@ -1,5 +1,9 @@
 include_directories("../libproxy")
 
 add_executable(proxy proxy.c)
-target_link_libraries(proxy libproxy)
+if (WITH_LIBRARY)
+    target_link_libraries(proxy libproxy)
+else ()
+    target_link_libraries(proxy PkgConfig::LIBPROXY)
+endif ()
 install(TARGETS proxy RUNTIME DESTINATION ${BIN_INSTALL_DIR})
