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.

--- CMakeLists.txt.orig	2020-12-29 11:30:46 UTC
+++ CMakeLists.txt
@@ -26,6 +26,15 @@ else()
   set(CMAKE_INSTALL_CONFIG_NAME ${CMAKE_BUILD_TYPE})
 endif()
 
+# Make building the library optional.
+option(WITH_LIBRARY "Build libproxy.so" ON)
+
+if (NOT WITH_LIBRARY)
+    # For the other ports provide an easy way to link against libproxy.so via PkgConfig::LIBPROXY.
+    find_package(PkgConfig)
+    pkg_check_modules(LIBPROXY libproxy-1.0 REQUIRED IMPORTED_TARGET)
+endif ()
+
 include(cmake/cpack.cmk)
 include(cmake/ctest.cmk)
 include(cmake/paths.cmk)
@@ -44,5 +53,7 @@ endif()
 
 # Always build these
 add_subdirectory(libproxy)
-add_subdirectory(utils)
+if (WITH_LIBRARY)
+    add_subdirectory(utils)
+endif()