fix: correct check for storage vs. rest enabled (#8855)

We cannot enable the storage library and disable the `rest-internal`
library, unfortunately the check was based on the wrong variables.

Author: coryan

CHANGELOG.md

@@ -84,6 +84,10 @@ For status on this, see https://github.com/googleapis/google-cloud-cpp/issues/88
 
 ## v1.41.0 - TBD
 
+## v1.40.1 - 2022-05
+
+* fix: correct check for storage vs. rest enabled ([#8850](https://github.com/googleapis/google-cloud-cpp/pull/8850))
+
 ## v1.40.0 - 2022-05
 
 We are happy to announce that the following GA libraries.  Unless specifically

CMakeLists.txt

@@ -272,13 +272,19 @@ cmake_dependent_option(
     ON "GOOGLE_CLOUD_CPP_ENABLE_REST_EXPRESSION" OFF)
 mark_as_advanced(GOOGLE_CLOUD_CPP_ENABLE_REST)
 
-# Prevent users for setting incompatible flags.
-if (GOOGLE_CLOUD_CPP_ENABLE_STORAGE AND NOT GOOGLE_CLOUD_CPP_ENABLE_REST)
+# Detect incompatible flags and let the customer know what to do.
+if (storage IN_LIST GOOGLE_CLOUD_CPP_ENABLE AND NOT
+                                                GOOGLE_CLOUD_CPP_ENABLE_REST)
     message(
         FATAL_ERROR
-            "If storage is enabled (e.g. -DGOOGLE_CLOUD_ENABLE=storage), "
-            "the REST library cannot be disabled "
-            "(e.g. -DGOOGLE_CLOUD_CPP_ENABLE_REST=OFF)")
+            "If storage is enabled (e.g. -DGOOGLE_CLOUD_CPP_ENABLE=storage),"
+            " the REST library cannot be disabled,"
+            " i.e., you cannot set -DGOOGLE_CLOUD_CPP_ENABLE_REST=OFF."
+            " If you do want to use the storage library,"
+            " then do not use -DGOOGLE_CLOUD_CPP_ENABLE_REST=OFF."
+            " If you do not want to use the storage library,"
+            " then provide a -DGOOGLE_CLOUD_CPP_ENABLE=... list and do not"
+            " include storage in that list.")
 endif ()
 
 # Enable testing in this directory so we can do a top-level `make test`. This