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Makefile.pre.in contains extra slash before $(DESTDIR) in two locations
as in the following:
sharedinstall:
 $(RUNSHARED) ./$(BUILDPYTHON) -E $(srcdir)/setup.py install \
 --prefix=$(prefix) \
 --install-scripts=$(BINDIR) \
 --install-platlib=$(DESTSHARED) \
 --root=/$(DESTDIR)
This causes Cygwin builds to fail if DESTDIR is set as follows:
creating //tmp
error: could not create '//tmp': No such host or network path
The following Open Group Specification:
http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html
indicates the following:
4.11 Pathanme Resolution 
[snip]
A pathname that begins with two successive slashes may be interpreted
in an implementation-defined manner,...
IMO, these extra slashes should be removed as indicated in the
attached patch. OK to commit? If so, to which branches?

I would like to confirm issue for trunk.

I'm not sure that installation work if slash is removed and DESTDIR is
not specified. What about to replace "slash before $(DESTDIR)" with "/./" ?

Sorry for the delay, but I was on vacation and then recovering from 
vacation...
You are correct that my initial patch does not handle the case when 
DESTDIR is not specified. Your suggestion will work, but seems hacky.
What about adding the defaulting of DESTDIR to "/" to my initial 
patch? This seems cleaner. See my second patch for the details.
If this approach is acceptable, then feel free to move the defaulting 
of DESTDIR to a more appropriate place in Makefile.pre.in. I wasn't 
sure where to put this change.

Jason, did you test what is result is DESTDIR default to "/".
This variable(macro) is used with specific syntax by example (from
Makefile):
 $(INSTALL) -d -m $(DIRMODE) $(DESTDIR)$$b; \
where $$b is absolute path.

Obviously not... :,(
It seems like we might have to go with your "/./" workaround, but let 
me see if I can come up with another approach.

Your latest patch is on the right track, but see my latest one, 
Makefile.pre.in.v2.diff, for an optimization. I tested make install 
with and without DESTDIR defined on the command line and both cases 
behaved as expected.

Hi Jason,
quick review without tests - I expect $${DESTDIR:-/} to work without :
in expression, i.e. just $${DESTDIR-/}.
The ":" in expression is not so portable (some shells fail on this).
Please see "Shell Substitutions" in autoconf manual.

I guess you mean the following:
${var:-value} 
Old BSD shells, including the Ultrix sh, don't accept the colon for 
any shell substitution, and complain and die.
If so, should we just go forward without the colon? The bash manpage 
indicates the following:
In each of the cases below, ... bash tests for a parameter that is 
unset or null; omitting the colon results in a test only for a 
parameter that is unset.
So, if a user executes "make DESTDIR= install", then the build will 
fail. Or, maybe we shouldn't worry about that corner case.
What do you think?

No response from py-dev list :( . May be solution has to work in all cases.

Is this still a problem with cygwin?

AFAICT, yes.