[CMake] Deprecate the rpath option and always append relative rpath

Since ROOT 6.36, the relative `RPATH` to the main ROOT libraries is only
appended to the existing path, and not replacing the path that is e.g.
passed by the user via `CMAKE_INSTALL_RPATH`. Therefore, there is no
problematic side effect of `rpath=ON` anymore. We can drop this option
and always append to the runpath.

This makes the ROOT configuration less confusing, also because from the
option name "rpath", it is not clear what it did.

Author: guitargeek

README/ReleaseNotes/v638/index.md

@@ -7,6 +7,9 @@
 * The `ROOT::Math::TDataPointN` class that can be used with the `ROOT::Math::KDETree` was removed. Use the templated `TDataPoint<N>` instead.
 * The Parallel ROOT Facility, `PROOF`, has been removed from the repository.
 * After being deprecated for a long period, the `-r` option of `rootcling` has been removed.
+* The `rpath` build option is deprecated. It is ignored and will result in configuration errors in the upcoming ROOT 6.38.
+  Relative RPATHs to the main ROOT libraries are unconditionally appended to all ROOT executables and libraries if the operating system supports it.
+  If you want a ROOT build without RPATHs, use the canonical CMake variable `CMAKE_SKIP_INSTALL_RPATH=TRUE`
 
 ## Core Libraries
 * Behavior change: when selecting a template instantiation for a dictionary, all the template arguments have to be fully defined - the forward declarations are not enough any more. The error prompted by the dictionary generator will be `Warning: Unused class rule: MyTemplate<MyFwdDeclaredClass>`.

cmake/modules/RootBuildOptions.cmake

@@ -154,7 +154,6 @@ ROOT_BUILD_OPTION(r OFF "Enable support for R bindings (requires R, Rcpp, and RI
 ROOT_BUILD_OPTION(roofit ON "Build the advanced fitting package RooFit, and RooStats for statistical tests. If xml is available, also build HistFactory.")
 ROOT_BUILD_OPTION(roofit_multiprocess OFF "Build RooFit::MultiProcess and multi-process RooFit::TestStatistics classes (requires ZeroMQ >= 3.4.5 built with -DENABLE_DRAFTS and cppzmq).")
 ROOT_BUILD_OPTION(root7 ON "Build ROOT 7 components of ROOT")
-ROOT_BUILD_OPTION(rpath ON "Link libraries with built-in RPATH (run-time search path)")
 ROOT_BUILD_OPTION(runtime_cxxmodules ON "Enable runtime support for C++ modules")
 ROOT_BUILD_OPTION(shadowpw OFF "Enable support for shadow passwords")
 ROOT_BUILD_OPTION(shared ON "Use shared 3rd party libraries if possible")
@@ -307,7 +306,6 @@ if(WIN32)
   set(davix_defvalue OFF)
   set(roofit_multiprocess_defvalue OFF)
   set(roottest_defvalue OFF)
-  set(rpath_defvalue OFF)
   set(runtime_cxxmodules_defvalue OFF)
   set(testing_defvalue OFF)
   set(vdt_defvalue OFF)
@@ -406,6 +404,15 @@ foreach(opt )
   endif()
 endforeach()
 
+if(DEFINED rpath)
+  message(DEPRECATION ">>> Option 'rpath' is deprecated and is ignored."
+      " Relative RPATHs to the main ROOT libraries are unconditionally appended to all ROOT"
+      " executables and libraries. The 'rpath' flag presence will result in a configuration"
+      " error in the next release of ROOT."
+      " Please contact root-dev@cern.ch should you still need this option."
+      "") # empty line at the end to make the deprecation message more visible
+endif()
+
 foreach(opt minuit2_mpi)
   if(${opt})
       message(WARNING "The option '${opt}' can only be used to minimise thread-safe functions in Minuit2. It cannot be used for Histogram/Graph fitting and for RooFit. If you want to use Minuit2 with MPI support, it is better to build Minuit2 as a standalone library.")

cmake/modules/RootMacros.cmake

@@ -975,7 +975,7 @@ function(ROOT_LINKER_LIBRARY library)
 
   #----Installation details-------------------------------------------------------
   if(NOT ARG_TEST AND NOT ARG_NOINSTALL AND CMAKE_LIBRARY_OUTPUT_DIRECTORY)
-    if(rpath)
+    if(NOT MSVC)
       ROOT_APPEND_LIBDIR_TO_INSTALL_RPATH(${library} ${CMAKE_INSTALL_LIBDIR})
     endif()
     if(ARG_CMAKENOEXPORT)
@@ -1440,7 +1440,7 @@ function(ROOT_EXECUTABLE executable)
   endif()
   #----Installation details------------------------------------------------------
   if(NOT ARG_NOINSTALL AND CMAKE_RUNTIME_OUTPUT_DIRECTORY)
-    if(rpath)
+    if(NOT MSVC)
       ROOT_APPEND_LIBDIR_TO_INSTALL_RPATH(${executable} ${CMAKE_INSTALL_BINDIR})
     endif()
     if(ARG_CMAKENOEXPORT)

rootx/CMakeLists.txt

@@ -28,6 +28,6 @@ target_include_directories(root PRIVATE
   ${CMAKE_BINARY_DIR}/ginclude         # for RConfigure.h and rootCommandLineOptionsHelp.h
 )
 
-if(rpath)
+if(NOT MSVC)
   target_compile_definitions(root PRIVATE IS_RPATH_BUILD)
 endif()