hadrian: Warn user if hadrian build fails due to lack of threaded RTS

See #16873.

hadrian/build.cabal.sh

@@ -21,13 +21,27 @@ fi
 CABVERSTR=$("$CABAL" --numeric-version)
 CABVER=( ${CABVERSTR//./ } )
 
+build_failed() {
+    ( ghc --info | grep -s '("Support SMP","YES")' ) \
+      || cat <<EOF
+Your compiler does not support the threaded runtime system.
+Please disable the \`threaded\` Cabal flag in project.cabal.local
+by running:
+
+    echo -e "package hadrian\n  flags: -threaded" >> project.cabal.local
+
+EOF
+    exit 1
+}
+
 if [ "${CABVER[0]}" -gt 2 -o "${CABVER[0]}" -eq 2 -a "${CABVER[1]}" -ge 2 ];
 then
     "$CABAL" --project-file="$PROJ" new-build $CABFLAGS -j exe:hadrian
     # use new-exec instead of new-run to make sure that the build-tools (alex & happy) are in PATH
     "$CABAL" --project-file="$PROJ" new-exec  $CABFLAGS    hadrian -- \
         --directory "$PWD" \
-        "$@"
+        "$@" \
+        || build_failed
 else
     echo "Cabal version is too old; you need at least cabal-install 2.2"
     exit 2

hadrian/hadrian.cabal

@@ -14,6 +14,15 @@ source-repository head
     type:     git
     location: https://gitlab.haskell.org/ghc/ghc
 
+-- To accomodate #16873
+flag threaded
+    manual:              True
+    default:             True
+    description:         Build with the threaded runtime for improved
+                         performance. Users with bootstrap compilers
+                         which don't support the threaded runtime should
+                         disable this flag.
+
 executable hadrian
     main-is:             Main.hs
     hs-source-dirs:      .
@@ -142,6 +151,9 @@ executable hadrian
                        -Wredundant-constraints
                        -fno-warn-name-shadowing
                        -rtsopts
+
+    if flag(threaded)
+      ghc-options:
                        -- * -I0: Disable idle GC to avoid redundant GCs while
                        --        waiting for external processes
                        -- * -qg: Don't use parallel GC as the synchronization