use shell variable CcLlvmBackend for test

Previously we used `AC_DEFINE`d variable `CC_LLVM_BACKEND` which has an empty shell expansion.

(cherry picked from commit 7cfb9baf)

configure.ac

@@ -1405,7 +1405,7 @@ echo "\
       which is version   : $GhcVersion
 "
 
-if test "x$CC_LLVM_BACKEND" = "x1"; then
+if test "x$CcLlvmBackend" = "xYES"; then
     CompilerName="clang        "
 else
     CompilerName="gcc          "