[master] 146a28651 symlink documentation source files from $(srcdir) to $(builddir)

[Nils Goroll]

commit 146a28651a140db54b8efd576e971efcd6cc3ae3
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Fri May 8 19:37:16 2020 +0200

    symlink documentation source files from $(srcdir) to $(builddir)
    
    ... if they differ.
    
    Ref #3309

diff --git a/doc/sphinx/Makefile.am b/doc/sphinx/Makefile.am
index fa71cacc9..ea010ab1a 100644
--- a/doc/sphinx/Makefile.am
+++ b/doc/sphinx/Makefile.am
@@ -30,9 +30,13 @@ clean:
 # use index.rst as an indicator if we have copied already
 .PHONY: link_srcdir
 link_srcdir:
-	if test "x$(srcdir)" != "x$(builddir)" && test ! -f index.rst ; then \
-		d=`pwd`/$(builddir) ; \
-		cd $(srcdir) && find . -type f | cpio -dmp $${d} || true ; \
+	if test "x$(srcdir)" != "x$(builddir)" && test ! -f index.rst; then \
+		s=`realpath $(srcdir)`; \
+		for f in `cd $$s && find . -type f`; do \
+			d=`dirname $$f`; \
+			test -d $$d || mkdir -p $$d; \
+			test -f $$f || ln -s $$s/$$f $$f; \
+		done \
 	fi
 
 # work around for make html called within doc/sphinx