add an explanation of the semi-colon on Makefile rule

diff --git a/tclpkg/gv/Makefile.am b/tclpkg/gv/Makefile.am
index 14d7221f690f5b95b9f8761959f278893a8f040d..42e1193198a90163fc0d43900261b924b539cee2 100644 (file)
--- a/tclpkg/gv/Makefile.am
+++ b/tclpkg/gv/Makefile.am
@@ -433,6 +433,8 @@ else
 pdf =
 endif
 
+# this rule has a trailing semi-colon for the Make cache to work correctly
+# https://stackoverflow.com/questions/37873522/unreliable-parallel-builds-in-a-makefile-with-intermediate
 $(man): gv_doc_writer.intermediate ;
 
 .INTERMEDIATE: gv_doc_writer.intermediate