enable parallel ghc for building git-annex

Via a build flag this time, that's off by default because hackage
demands it be so, but that gets turned on by the Makefile and by stack.

Makefile

@@ -35,7 +35,7 @@ install-home:
 tmp/configure-stamp: Build/TestConfig.hs Build/Configure.hs
 	if [ "$(BUILDER)" = ./Setup ]; then $(GHC) --make Setup; fi
 	if [ "$(BUILDER)" != stack ]; then \
-		$(BUILDER) configure $(BUILDERCOMMONOPTIONS) --ghc-options="$(shell Build/collect-ghc-options.sh)"; \
+		$(BUILDER) configure -fParallelBuild $(BUILDERCOMMONOPTIONS) --ghc-options="$(shell Build/collect-ghc-options.sh)"; \
 		rm cabal.project.local~* 2>/dev/null || true; \
 	else \
 		$(BUILDER) setup $(BUILDERCOMMONOPTIONS); \
@@ -53,7 +53,7 @@ dev:
 # This leaves cabal.project.local configured for a prof build,
 # so just running make will continue to do prof builds.
 prof:
-	$(BUILDER) configure -f"-Production" \
+	$(BUILDER) configure -f"-Production" -fParallelBuild \
 		--enable-executable-dynamic --enable-profiling
 	rm cabal.project.local~* 2>/dev/null || true
 	mkdir -p tmp

git-annex.cabal

@@ -159,6 +159,10 @@ Flag Pairing
 Flag Production
   Description: Enable production build (slower build; faster binary)
 
+Flag ParallelBuild
+  Description: Enable production build (slower build; faster binary)
+  Default: False
+
 Flag TorrentParser
   Description: Use haskell torrent library to parse torrent files
 
@@ -292,6 +296,9 @@ Executable git-annex
   else
     GHC-Options: -O0
 
+  if flag(ParallelBuild)
+    GHC-Options: -j
+
   -- Avoid linking with unused dynamic libraries.
   if os(linux) || os(freebsd)
     GHC-Options: -optl-Wl,--as-needed

stack-lts-18.13.yaml

@@ -1,6 +1,7 @@
 flags:
   git-annex:
     production: true
+    parallelbuild: true
     assistant: true
     pairing: true
     torrentparser: true

stack.yaml

@@ -1,6 +1,7 @@
 flags:
   git-annex:
     production: true
+    parallelbuild: true
     assistant: true
     pairing: true
     torrentparser: true

standalone/linux/stack-i386ancient.yaml

@@ -1,6 +1,7 @@
 flags:
   git-annex:
     production: true
+    parallelbuild: true
     assistant: true
     pairing: true
     torrentparser: true