Error if source-dist target is run in out-of-tree build

Makefile

@@ -480,8 +480,10 @@ else
 endif
  rm -fr $(BUILDROOT)/julia-$(JULIA_COMMIT)
 
-# this target does not accept BUILDROOT
 light-source-dist.tmp: $(BUILDROOT)/doc/_build/html/en/index.html
+ifneq ($(BUILDROOT),$(JULIAHOME))
+ $(error make light-source-dist does not work in out-of-tree builds)
+endif
  # Save git information
  -@$(MAKE) -C $(JULIAHOME)/base version_git.jl.phony
 
@@ -491,7 +493,6 @@ light-source-dist.tmp: $(BUILDROOT)/doc/_build/html/en/index.html
  find doc/_build/html >> light-source-dist.tmp
 
 # Make tarball with only Julia code
-# this target does not accept BUILDROOT
 light-source-dist: light-source-dist.tmp
  # Prefix everything with the current directory name (usually "julia"), then create tarball
  DIRNAME=$$(basename $$(pwd)); \
@@ -502,7 +503,6 @@ source-dist:
  @echo \'source-dist\' target is deprecated: use \'full-source-dist\' instead.
 
 # Make tarball with Julia code plus all dependencies
-# this target does not accept BUILDROOT
 full-source-dist: light-source-dist.tmp
  # Get all the dependencies downloaded
  @$(MAKE) -C deps getall NO_GIT=1