[CMake] Fix ordering problem when checking and setting cuda flag

In the ROOT build system, `CheckCompiler` is included before
`RootBuildOptions`. The problem with this is that when building with
`-Dall=ON`, the `cuda` flag is only set in `RootBuildOptions`, which is
too late to find the CUDA compiler in `CheckCompiler`. This results in
issues like root-project#15139:
```
CMake Error in roofit/batchcompute/CMakeLists.txt:
  Exporting the target "RooBatchCompute_CUDA" is not allowed since its linker
  language cannot be determined
```

This commit suggests a solution to this problem: just always enable the
CUDA language in CMake if it is found, and only set the `cuda` flag with
`-Dall=ON` if the CUDA language is enabled.

cmake/modules/CheckCompiler.cmake

@@ -51,18 +51,14 @@ else()
   set(CMAKE_Fortran_COMPILER CMAKE_Fortran_COMPILER-NOTFOUND)
 endif()
 
-#---Enable CUDA ---
-if(cuda)
-  check_language(CUDA)
-  if (CMAKE_CUDA_COMPILER)
-    enable_language(CUDA)
-    if(NOT DEFINED CMAKE_CUDA_STANDARD)
-      set(CMAKE_CUDA_STANDARD ${CMAKE_CXX_STANDARD})
-    endif()
-  else()
-      message(FATAL_ERROR "CUDA not found. Ensure that the installation of CUDA is in the CMAKE_PREFIX_PATH, or disable CUDA features in ROOT with -Dcuda=OFF")
+#---Enable CUDA if found on the system---
+check_language(CUDA)
+if (CMAKE_CUDA_COMPILER)
+  enable_language(CUDA)
+  if(NOT DEFINED CMAKE_CUDA_STANDARD)
+    set(CMAKE_CUDA_STANDARD ${CMAKE_CXX_STANDARD})
   endif()
-endif(cuda)
+endif()
 
 #----Test if clang setup works----------------------------------------------------------------------
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")

cmake/modules/RootBuildOptions.cmake

@@ -219,7 +219,6 @@ if(all)
  set(asimage_defvalue ON)
  set(cefweb_defvalue ON)
  set(clad_defvalue ON)
- set(cuda_defvalue ON)
  set(dataframe_defvalue ON)
  set(test_distrdf_pyspark_defvalue ON)
  set(test_distrdf_dask_defvalue ON)
@@ -255,7 +254,6 @@ if(all)
  set(ssl_defvalue ON)
  set(tmva_defvalue ON)
  set(tmva-cpu_defvalue ON)
- set(tmva-gpu_defvalue ON)
  set(tmva-pymva_defvalue ON)
  set(tmva-rmva_defvalue ON)
  set(unuran_defvalue ON)
@@ -266,6 +264,10 @@ if(all)
  set(x11_defvalue ON)
  set(xml_defvalue ON)
  set(xrootd_defvalue ON)
+ if(CMAKE_CUDA_COMPILER)
+   set(cuda_defvalue ON)
+   set(tmva-gpu_defvalue ON)
+ endif()
 endif()
 
 #--- The 'builtin_all' option switches ON all the built in options but GPL-------------------------------
@@ -367,11 +369,6 @@ if(roottest OR rootbench)
   set(testing ON CACHE BOOL "" FORCE)
 endif()
 
-#---tmva-gpu option implies cuda
-if(tmva-gpu)
-  set(cuda ON CACHE BOOL "" FORCE)
-endif(tmva-gpu)
-
 if(cudnn AND NOT cuda)
   message(STATUS "Cannot enable cudnn without enabling cuda or tmva-gpu: cudnn is disabled.")
   set(cudnn OFF)