[CMake] LLVM NVPTX target only if 'cuda' is enabled

CUDA in Cling is an optional feature that we know doesn't properly work from inside ROOT (with modules).

Author: LukasBreitwieser

interpreter/CMakeLists.txt

@@ -92,7 +92,7 @@ else()
   set(ROOT_CLING_TARGET "all")
 endif()
 
-if(NOT "${ROOT_CLING_TARGET}" STREQUAL "all")
+if(cuda AND NOT "${ROOT_CLING_TARGET}" STREQUAL "all")
   string(APPEND ROOT_CLING_TARGET ";NVPTX")
 endif()
 

interpreter/cling/CMakeLists.txt

@@ -118,10 +118,6 @@ else()
   endif()
 endif()
 
-if( NOT "NVPTX" IN_LIST LLVM_TARGETS_TO_BUILD)
-  message(FATAL_ERROR "NVPTX backend is not activated\n"
-    "Please enable it via -DLLVM_TARGETS_TO_BUILD=\"host;NVPTX\"")
-endif()
 
 set(CLING_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(CLING_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})

interpreter/cling/lib/Interpreter/IncrementalCUDADeviceCompiler.cpp

@@ -16,6 +16,7 @@
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Lex/HeaderSearchOptions.h"
 
+#include "llvm/Config/llvm-config.h" // for LLVM_HAS_NVPTX_TARGET
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/raw_ostream.h"
@@ -93,13 +94,15 @@ namespace cling {
       return;
     }
 
+#if LLVM_HAS_NVPTX_TARGET
     // initialize NVPTX backend
     LLVMInitializeNVPTXTargetInfo();
     LLVMInitializeNVPTXTarget();
     LLVMInitializeNVPTXTargetMC();
     LLVMInitializeNVPTXAsmPrinter();
 
     m_Init = true;
+#endif // LLVM_HAS_NVPTX_TARGET
   }
 
   void IncrementalCUDADeviceCompiler::setCuArgs(