Simplify Configure in a few ways

 - No need to distinguish between gcc-llvm and clang. First of all,
   gcc-llvm is quite old and surely unmaintained by now. Second of all,
   none of the code actually care about that distinction!

   Now, it does make sense to consider C multiple frontends for LLVMs in
   the form of clang vs clang-cl (same clang, yes, but tweaked
   interface). But this is better handled in terms of "gccish vs
   mvscish" and "is LLVM", yielding 4 combinations. Therefore, I don't
   think it is useful saving the existing code for that.

 - Get the remaining CC_LLVM_BACKEND, and also TABLES_NEXT_TO_CODE in
   mk/config.h the normal way, rather than hacking it post-hoc. No point
   keeping these special cases around for now reason.

 - Get rid of hand-rolled `die` function and just use `AC_MSG_ERROR`.

 - Abstract check + flag override for unregisterised and tables next to
   code.

Oh, and as part of the above I also renamed/combined some variables
where it felt appropriate.

 - GccIsClang -> CcLlvmBackend. This is for `AC_SUBST`, like the other
 Camal case ones. It was never about gcc-llvm, or Apple's renamed clang,
 to be clear.

 - llvm_CC_FLAVOR -> CC_LLVM_BACKEND. This is for `AC_DEFINE`, like the
 other all-caps snake case ones. llvm_CC_FLAVOR was just silly
 indirection *and* an odd name to boot.

1 files changed, 1 insertions, 1 deletions

diff --git a/rts/sm/GCThread.h b/rts/sm/GCThread.h
index 66f604d6cc..66f7a7f84f 100644
--- a/rts/sm/GCThread.h
+++ b/rts/sm/GCThread.h
@@ -208,7 +208,7 @@ extern uint32_t n_gc_threads;
 
 extern gc_thread **gc_threads;
 
-#if defined(THREADED_RTS) && defined(llvm_CC_FLAVOR)
+#if defined(THREADED_RTS) && defined(CC_LLVM_BACKEND)
 extern ThreadLocalKey gctKey;
 #endif