By default, be lax about dependencies on GHC

There are a number of things which technically depend on GHC (e.g. if
ghc changes then Haskell files may be compiled differently, or Cabal
packages may be configured differently). However, in practice, having
a real dependency on GHC is just a pain: We normally don't want to
spend time recompiling other things while we're working on the
compiler, and even if we did, GHC will normally decide compilation
isn't needed anyway. So by default we use order-only dependencies on
GHC, i.e. GHC must exist, but if it's newer than other targets then
rebuilding is not necessary.

1 files changed, 19 insertions, 0 deletions

diff --git a/ghc.mk b/ghc.mk
index bae8d91273..53ad19d5a0 100644
--- a/ghc.mk
+++ b/ghc.mk
@@ -284,6 +284,25 @@ include rules/bindist.mk
 %/. : | $(MKDIRHIER)
 	"$(MKDIRHIER)" $@
 
+# -----------------------------------------------------------------------------
+# Lax dependencies
+
+ifeq "$(LAX_DEPENDENCIES)" "YES"
+LAX_DEPS_FOLLOW = |
+else
+LAX_DEPS_FOLLOW =
+endif
+
+# This is a bit of a hack. When LAX_DEPS_FOLLOW is | some rules end up
+# looking like
+#     target: a | b | c
+# The first | signals the start of the order-only dependencies, but make
+# treats the second | as a dependency. So we need to tell make how to
+# build that dependency.
+
+.PHONY: |
+| :
+	@:
 
 # -----------------------------------------------------------------------------
 # Properties of packages