Fix for cross compilation

In 8.0, PostgreSQL lost its previously postulated ability to be
cross-compiled and it turns out some people actually used that, so
here's an attempt to fix it.

The problem is that the program zic in src/timezone/ is built and run
during the build process, which doesn't work if it's compiled by a
cross-compiler.

The standard way to go about this in autotools land (references: gcc,
binutils) is to introduce a second variable CC_FOR_BUILD that is set to
a native compiler. There is no particular way to determine this
variable; it's passed by the user or defaults to $CC. This variable is
then used in place of CC to build programs that are compiled and run
during the build.

The problem is that native compilations cannot rely on things that the
build system normally provides because the tests are run for the target
system, not the build system. It can be assumed that the compilers are
of the same kind, so using CFLAGS etc. is OK. But one cannot rely on
libraries for example without entering a world of pain. Since most
programs that need native compilation are usually some small conversion
programs, this generally works out well anyway.

Attached is a patch that implements that principle for zic. zic can no
longer use libpgport, but the only thing it seems to use from there is
strerror() and I like to think that we can get away with that.

I haven't actually tried this out with a cross compilation since I'm not
set up for it, so if someone has an environment available, please give
this a try.

Comments?

--
Peter Eisentraut
http://developer.postgresql.org/~petere/