fix cross-compilation support

fix cross-compilation support

Commit 2500ebfc89 (util: fix make dependencies for subst) broke cross
compilation because it unconditionally used config.h without setting a
includes path so that the config.h file could be found.

The proposed fix of adding the include path (such as was proposed at
http://patchwork.ozlabs.org/patch/355662/ or in Debian Bug #753375)
isn't really the right way to go, since the information in config.h is
for the target environment, and not the build environment.  So using
config.h when building helper programs used as part of the build can
potentially cause more problems than it solves.

In general, build helpers must be written to be as portable as
possible, and to not require any autoconf defined #ifdef's whenever
possible.  The subst program broke this rule to (1) address a Coverity
security complaint by using futimes(2) instad of utimes(2) if present,
and (2) to preserve the nanosecond portion of the file timestamp.

Oh, well.  We won't be able to do the latter when cross compiling, and
as to the former, if an attacker has write access to your build tree
while you are building programs that will be run as root, you've got
bigger problems.  :-)

Fix the problem that commit 2500ebfc89 was trying to address by
explicitly adding @DEFS@ to CFLAGS, so that -DHAVE_CONFIG_H is passed
to make depend.  This fixes up the make depend without forcing the use
of config.h when cross-compiling.

Addresses-Debian-Bug: #753375
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Helmut Grohne <helmut@subdivi.de>
Cc: 753375@bugs.debian.org