Log LLVM library version in configure output.

When scanning build farm results, it's useful to be able to see which
version is in use.  For the Meson build system, this information was
already displayed.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/4022690.1697852728%40sss.pgh.pa.us

config/llvm.m4

@@ -28,6 +28,7 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
   if echo $pgac_llvm_version | $AWK -F '.' '{ if ([$]1 >= 4 || ([$]1 == 3 && [$]2 >= 9)) exit 1; else exit 0;}';then
     AC_MSG_ERROR([$LLVM_CONFIG version is $pgac_llvm_version but at least 3.9 is required])
   fi
+  AC_MSG_NOTICE([using llvm $pgac_llvm_version])
 
   # need clang to create some bitcode files
   AC_ARG_VAR(CLANG, [path to clang compiler to generate bitcode])

configure

@@ -5123,6 +5123,8 @@ fi
   if echo $pgac_llvm_version | $AWK -F '.' '{ if ($1 >= 4 || ($1 == 3 && $2 >= 9)) exit 1; else exit 0;}';then
     as_fn_error $? "$LLVM_CONFIG version is $pgac_llvm_version but at least 3.9 is required" "$LINENO" 5
   fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: using llvm $pgac_llvm_version" >&5
+$as_echo "$as_me: using llvm $pgac_llvm_version" >&6;}
 
   # need clang to create some bitcode files