Fix find module not working when no version number was given

cmake/protobuf-config-version.cmake.in

@@ -17,20 +17,22 @@ endif()
 
 set(PACKAGE_VERSION_COMPATIBLE TRUE) #Assume true until shown otherwise
 
-if(NOT PACKAGE_FIND_VERSION_MAJOR EQUAL "@protobuf_VERSION_MAJOR@")
-  set(PACKAGE_VERSION_COMPATIBLE FALSE)
-elseif(PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
-  set(PACKAGE_VERSION_COMPATIBLE FALSE)
-elseif(PACKAGE_FIND_VERSION VERSION_EQUAL PACKAGE_VERSION)
-  # Do not match prerelease versions to non-prerelease version requests.
-  if(NOT "@protobuf_VERSION_PRERELEASE@" STREQUAL "" AND PACKAGE_FIND_VERSION_PRERELEASE STREQUAL "")
-    message(AUTHOR_WARNING "To use this prerelease version of ${PACKAGE_FIND_NAME}, set ${PACKAGE_FIND_NAME}_FIND_VERSION_PRERELEASE to '@protobuf_VERSION_PRERELEASE@' or greater.")
+if(PACKAGE_FIND_VERSION) #Only perform version checks if one is given
+  if(NOT PACKAGE_FIND_VERSION_MAJOR EQUAL "@protobuf_VERSION_MAJOR@")
     set(PACKAGE_VERSION_COMPATIBLE FALSE)
-  endif()
-
-  # Not robustly SemVer compliant, but protobuf never uses '.' separated prerelease identifiers.
-  if(PACKAGE_FIND_VERSION_PRERELEASE STRGREATER "@protobuf_VERSION_PRERELEASE@")
+  elseif(PACKAGE_FIND_VERSION VERSION_GREATER PACKAGE_VERSION)
     set(PACKAGE_VERSION_COMPATIBLE FALSE)
+  elseif(PACKAGE_FIND_VERSION VERSION_EQUAL PACKAGE_VERSION)
+    # Do not match prerelease versions to non-prerelease version requests.
+      if(NOT "@protobuf_VERSION_PRERELEASE@" STREQUAL "" AND PACKAGE_FIND_VERSION_PRERELEASE STREQUAL "")
+      message(AUTHOR_WARNING "To use this prerelease version of ${PACKAGE_FIND_NAME}, set ${PACKAGE_FIND_NAME}_FIND_VERSION_PRERELEASE to '@protobuf_VERSION_PRERELEASE@' or greater.")
+      set(PACKAGE_VERSION_COMPATIBLE FALSE)
+    endif()
+
+    # Not robustly SemVer compliant, but protobuf never uses '.' separated prerelease identifiers.
+    if(PACKAGE_FIND_VERSION_PRERELEASE STRGREATER "@protobuf_VERSION_PRERELEASE@")
+      set(PACKAGE_VERSION_COMPATIBLE FALSE)
+    endif()
   endif()
 endif()