Merge pull request #12321 from bartlettroscoe/12104-improve-gnuinstal…

…ldirs-release-notes

Improve release note for using GNUInstallDirs by default (#12104)

RELEASE_NOTES

@@ -8,24 +8,41 @@
 CMake
 
   - Change the default for `Trilinos_MUST_FIND_ALL_TPL_LIBS` from `OFF` to
-    `ON`. It was
-    turned off by default because it breaks backward compatibility but it also
-    causes problems for new users and new configurations.
+    `ON`. It was turned off by default because it breaks backward
+    compatibility but it also causes problems for new users and new
+    configurations.
 
     Users that do not want this new behavior can set
     `-D Trilinos_MUST_FIND_ALL_TPL_LIBS=OFF`, which is backward compatible.
 
   - Change the default for `Trilinos_USE_GNUINSTALLDIRS` from `OFF` to `ON`,
-    in the goal to
-    move Trilinos and TriBITS to modern CMake.
+    in the goal to move Trilinos and TriBITS to modern CMake.
 
     TriBITS has had the ability to use that paths selected by the standard
     CMake module `GNUInstallDirs.cmake` for a long time. But it is turned off
     in TriBITS by default and was never turned on in Trilinos, both for the
     sake of backward compatibility.
 
     This may break people's existing configurations because it will install
-    libs in `<prefix>/libs64/` instead of in `<instead>/libs/`. See
+    libs in `<prefix>/libs64/` instead of in `<prefix>/libs/` on many systems
+    (e.g. Linux systems).  For example, this will break downstream CMake
+    projects that call `find_package(Trilinos ...)` before defining the
+    compilers (e.g. so they can get the compilers from Trilinos).  If the
+    compilers are not defined, `find_package()` will not search
+    `<prefix>/lib64`.  To revert back to using `<prefix>/lib` but still use
+    `GNUInstallDirs.cmake` for Trilinos, set `-D
+    CMAKE_INSTALL_LIBDIR:STRING=lib` when configuring Trilinos.  To avoid the
+    `find_package(Trilinos ...)` problem not searching `<prefix>/lib64`,
+    consider explicitly specifying the compiler to and having the downstream
+    CMake project define the compilers first with `project(<ProjectName>
+    COMPILERS C CXX ...)` before calling `find_package(Trilinos ...)`.  (That
+    is, don't try to get the compilers from the installed Trilinos, see
+    https://github.com/trilinos/Trilinos/issues/12306.)
+
+    NOTE: The setting `-D Trilinos_USE_GNUINSTALLDIRS=OFF` is deprecated and
+    may be removed in the future. (I.e. the usage of `GNUInstallDirs.cmake`
+    may be hard-coded in the future so please try adjusting to the usage of
+    `GNUInstallDirs.cmake` by Trilinos.)  See
     https://github.com/trilinos/Trilinos/issues/12104#issuecomment-1691945033
     for additional details and instructions.