docs: fix intermittent make distcheck failures

Use .NOTPARALLEL when building docs. This avoids intermittent
make distcheck failures like 'cp: cannot create regular file
`build/image.entities': File exists' when using -jN.

Fixes #590718.

diff --git a/docs/manuals.mak b/docs/manuals.mak
index f4497de..353c44c 100644 (file)
--- a/docs/manuals.mak
+++ b/docs/manuals.mak
@@ -173,3 +173,7 @@ $(BUILDIMAGESDIR)/%.ps: %.png
 check-local: $(BUILDDIR)/$(MAIN)
        @cp -f $(srcdir)/../image-png $(BUILDDIR)/image.entities
        cd $(BUILDDIR) && xmllint -noout -valid $(MAIN)
+
+# avoid 'cp: cannot create regular file `build/image.entities': File exists'
+# errors during 'make distcheck' by disabling parallel builds
+.NOTPARALLEL: