cmake workarround for quark in older cmake where pkg config produces non existing targets

CMakeLists.txt

@@ -183,8 +183,15 @@ option(FORCE_OXENC_SUBMODULE "force using oxen-encoding submodule" OFF)
 if(NOT FORCE_OXENC_SUBMODULE)
   pkg_check_modules(OXENC liboxenc>=1.0.1 IMPORTED_TARGET)
 endif()
+
 if(OXENC_FOUND)
-  add_library(oxenc::oxenc ALIAS PkgConfig::OXENC)
+  if(NOT TARGET PkgConfig::OXENC AND CMAKE_VERSION VERSION_LESS "3.21")
+    # Work around cmake bug 22180 (PkgConfig::OXENC not set if no flags needed):
+    add_library(_empty_oxenc INTERFACE)
+    add_library(oxenc::oxenc ALIAS _empty_oxenc)
+  else()
+    add_library(oxenc::oxenc ALIAS PkgConfig::OXENC)
+  endif()
   message(STATUS "Found system liboxenc ${OXENC_VERSION}")
 else()
   message(STATUS "using oxen-encoding submodule")