Handle _CROW_ILL or _CROW_ICD being NOTFOUND

If the variable is NOTFOUND then set it to "". Then at the end after possible list appends. If the variable is not "" then update the target properties.

This gets rid of a warning

cmake/CrowConfig.cmake.in

@@ -28,6 +28,13 @@ check_required_components("@PROJECT_NAME@")
 get_target_property(_CROW_ILL Crow::Crow INTERFACE_LINK_LIBRARIES)
 get_target_property(_CROW_ICD Crow::Crow INTERFACE_COMPILE_DEFINITIONS)
 
+if(_CROW_ILL STREQUAL "_CROW_ILL-NOTFOUND")
+        set(_CROW_ILL "")
+endif()
+if(_CROW_ICD STREQUAL "_CROW_ICD-NOTFOUND")
+        set(_CROW_ICD "")
+endif()
+
 list(REMOVE_ITEM _CROW_ILL "ZLIB::ZLIB" "OpenSSL::SSL")
 list(REMOVE_ITEM _CROW_ICD "CROW_ENABLE_SSL" "CROW_ENABLE_COMPRESSION")
 
@@ -41,7 +48,13 @@ if("ssl" IN_LIST CROW_FEATURES)
   list(APPEND _CROW_ICD "CROW_ENABLE_SSL")
 endif()
 
-set_target_properties(Crow::Crow PROPERTIES
-  INTERFACE_COMPILE_DEFINITIONS "${_CROW_ICD}"
-  INTERFACE_LINK_LIBRARIES "${_CROW_ILL}"
-)
+if( NOT (_CROW_ICD STREQUAL "" ) )
+  set_target_properties(Crow::Crow PROPERTIES
+    INTERFACE_COMPILE_DEFINITIONS "${_CROW_ICD}"
+  )
+endif()
+if( NOT (_CROW_ILL STREQUAL "" ) )
+  set_target_properties(Crow::Crow PROPERTIES
+    INTERFACE_LINK_LIBRARIES "${_CROW_ILL}"
+  )
+endif()