[TCLCORE] Tcl autoconf: cross-building on mingw32/64

[Anton K. <an...@sw...>]

Hello Tcl developers,

There is a couple of issues with Tcl autoconf script for Windows, that
I routinely run across when doing cross-builds with mingw from Linux.
After posting a message about SEH, concerning the changes that require
autoconf modifications, I decided to initiate discussion on these
issues as well.

Cross-compilation is obviously _not_ what the tcl.m4 author(s) had in
mind. But, fortunately, this script works reasonbly well even in this
case, for building across different-CPU linuxes and for windwos as well,
with the following two exceptions:

1. System threading implementation selection seems to depend on
`uname` output unconditionally. When running on Linux, I can
cross-compile non-threaded Tcl/win32 and non-threaded extensions
successfully, but when attempting the threaded build, it tries to use
pthread and resorts to a non-threaded one.

Overriding the uname command by something that comes earlier in the
$PATH and echoes MINGW32_NT-5.1_WIN32 solves this issue, but this
solution is unobvious and probably causes many people to abandon the
idea of cross-compiling Tcl. It should be solved in the script
instead, but, again, I don't have enough autoconf experience to
provide a _reliable_ solution (probably the build system uname should
not be used at all).

2. When installing message catalogs and timezone data, Makefile
insists on running the freshly-built tclsh (that isn't generally
runnable in case of cross-compiling). For windows builds, I usually
leave it alone and let wine provide the transparent execution. For
cross-linux builds, I sometimes let qemu do the same. Before these
solutions started to work nicely (like they do now) I had to separate out 
install-binaries, install-libraries and (install-tzdata install-msgs),
running the last pair of targets with overridden TCLSH.

I propose a fix: let Makefile use $RUNNABLE_TCLSH in the installation
process, with $RUNNABLE_TCLSH defaulting to the freshly built $TCLSH, as
it was before. That is, it should _install_ $TCLSH into $(prefix)/bin,
but _run_ (when needed) $(RUNNABLE_TCLSH), which defaults to $(TCLSH)
but can be overridden separately.

This way, Tcl's configure script doesn't have to support finding the
appropriate build-system runnable tclsh. The modification is two lines
long. But when I build an automatically cross-compiled tree, it would
save much time to just "make install RUNNABLE_TCLSH=/usr/bin/tclsh
INSTALL_ROOT=zzzz" instead of fiddling with Makefile internals (and
merging the changes when they happen).

I realize that the cross-compilation support is probably not a
high-priority goal for Tcl team. But this target is almost reached
anyway, so it'd be nice to take a minor additional step :)

-- 
Regards,
Anton Kovalenko
+7(916)345-34-02
Elektrostal' MO, Russia