[cmake] make fail-on-missing=ON the only behavior, ignore user setting and deprecate build option

.github/workflows/root-ci-config/buildconfig/alma10-minimal-asan.txt

@@ -3,7 +3,6 @@ CMAKE_CXX_FLAGS_DEBUG="-Og -g"
 asan=ON
 ccache=ON
 builtin_vdt=ON
-fail-on-missing=ON
 minimal=ON
 roottest=ON
 testing=ON

.github/workflows/root-ci-config/buildconfig/alma10-minimal.txt

@@ -1,6 +1,5 @@
 ccache=ON
 builtin_vdt=ON
-fail-on-missing=ON
 minimal=ON
 roottest=ON
 testing=ON

.github/workflows/root-ci-config/buildconfig/global.txt

@@ -40,7 +40,6 @@ davix=ON
 dcache=OFF
 dev=OFF
 distcc=OFF
-fail-on-missing=ON
 fcgi=OFF
 fftw3=ON
 fitsio=ON

README/ReleaseNotes/v642/index.md

@@ -40,6 +40,7 @@ The following people have contributed to this new version:
 ## Deprecation and Removal
 
 * The build options `vc`, `veccore`, `builtin_vc`, `builtin_veccore` and `rpath` that were deprecated are now removed and will result in configuration errors if used.
+* The option `fail-on-missing` is deprected in 6.42 (it has no effect, now the behavior will be as if it was always ON), and will be removed in 6.44.
 * The method `RooRealVar::removeRange()` and the corresponding method in `RooErrorVar` that were deprecated in ROOT 6.40 are now removed.
 * The overloads of `RooAbsReal::createChi2()` and `RooAbsReal::chi2FitTo()` that take unbinned **RooDataSet** data objects were deprecated in ROOT 6.40 and are now removed.
 * The **RooStats::HybridPlot** class and the related **HybridResult::GetPlot** method were deprecated in ROOT 6.40 and are now removed.

cmake/modules/CheckCompiler.cmake

@@ -49,8 +49,8 @@ if(fortran)
       enable_language(Fortran)
     endif()
   endif()
-  if(NOT CMAKE_Fortran_COMPILER AND fail-on-missing)
-    message(FATAL_ERROR "No Fortran compiler found. Please make sure it's installed, or disable ROOT's Fortran features with '-Dfortran=OFF' (or set '-Dfail-on-missing=OFF' to automatically disable features with missing requirements)")
+  if(NOT CMAKE_Fortran_COMPILER)
+    message(SEND_ERROR "No Fortran compiler found. Please make sure it's installed, or disable ROOT's Fortran features with '-Dfortran=OFF'")
   endif()
 else()
   set(CMAKE_Fortran_COMPILER CMAKE_Fortran_COMPILER-NOTFOUND)

cmake/modules/RootBuildOptions.cmake

@@ -186,7 +186,6 @@ ROOT_BUILD_OPTION(xrootd ON "Enable support for XRootD file server and client")
 
 option(all "Enable all optional components by default" OFF)
 option(clingtest "Enable cling tests (Note: that this makes llvm/clang symbols visible in libCling)" OFF)
-option(fail-on-missing "Fail at configure time if a required package cannot be found" OFF)
 option(gminimal "Enable only required options by default, but include X11/Cocoa" OFF)
 option(minimal "Enable only required options by default" OFF)
 option(rootbench "Build rootbench if rootbench exists in root or if it is a sibling directory (implies testing=ON)" OFF)
@@ -389,6 +388,13 @@ foreach(opt r)
   endif()
 endforeach()
 
+if(DEFINED fail-on-missing)
+  message(DEPRECATION ">>> Option 'fail-on-missing' is deprecated and ignored."
+      " ROOT now behaves as if fail-on-missing was always ON."
+      " Using this option will result in configuration errors in ROOT 6.44."
+      "") # 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.")