Mark all targets as "secondary"

This allows make to avoid rebuilding unnecessary intermediate files, which
is useful when cross-compiling.

See Parser/pgen and Programs/_freeze_importlib in Makefile.pre.in.

diff --git a/Makefile.pre.in b/Makefile.pre.in
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -1672,6 +1672,8 @@
 .PHONY: smelly funny patchcheck touch altmaninstall commoninstall
 .PHONY: gdbhooks
 
+.SECONDARY:
+
 # IF YOU PUT ANYTHING HERE IT WILL GO AWAY
 # Local Variables:
 # mode: makefile