bug#48088: libtool hardcodes native (cross-)compiler, incorrect when using libtool as cross-compiler

Hi,

[CC'ing to libtool mailing list, maybe they can shed some light on the matter
of how libtool can be used as a cross-compiler.  This concerns the definition
of the libtool package in guix.]

$ cat $(guix build libtool)/bin/libtool
Some relevant output:

> # A C compiler.
> LTCC="gcc"
> # Compile-time system search path for libraries.
> sys_lib_search_path_spec="/gnu/store/8zvc5mvk0xm3ygrxsgpyy5ilxb5rzjry-perl-5.30.2/lib /gnu/store/i5a9qb07y9xjh9ry8hp6km43kji16nmd-libltdl-2.4.6/lib /gnu/store/i8h2pcxqdq07ijm3ibkka8f4smn1w48v-bzip2-
1.0.8/lib /gnu/store/9860f1abqj8wjjnwl8a9v54pdcc3bhgf-xz-5.2.4/lib /gnu/store/60g7r3l01fd7c58yjbm6krgcwj1jkpwg-file-5.38/lib /gnu/store/swqdvwri9dbv6zssg6v0by7l05hd6wxp-gawk-5.0.1/lib
/gnu/store/m1z7cdbqsqyp9xnjw5cvlb4a7gkcg3m4-binutils-2.34/lib /gnu/store/fa6wj5bxkj5ll1d7292a70knmyl7a0cr-glibc-2.31/lib /gnu/store/s3dcqzwqaakv1yx37by9chksdbkgih17-glibc-2.31-static/lib
/gnu/store/hwcky7446s952w0mwchhmm211ll07zrq-glibc-utf8-locales-2.31/lib /gnu/store/01b4w3m6mp55y531kyi1g8shh722kwqm-gcc-7.5.0-lib/lib/gcc/x86_64-unknown-linux-gnu/7.5.0
/gnu/store/01b4w3m6mp55y531kyi1g8shh722kwqm-gcc-7.5.0-lib/lib "

Usually, libtool uses the compiler it was passed.
However, during linking, (at least during the compilation of libtomsmath IIRC),
libtool uses LTCC instead.  Thus, when a package X has libtool in native-inputs,
and it is being cross-compiled, it should really use a variant of libtool that has
been configured to set LTCC=TARGET-gcc (or LTCC=TARGET-clang).

There do not seem to exist any such variants currently.  (For a relatively clean
method for automatically using a cross-compiling variant, see the "pkg-config"
macro.) Thus, I thought it would ‘simply’ be a matter of passing an appropriate
"--target=TARGET" option to libtool's configure script when libtool is being used
as cross-compiler.  But alas, libtool configure only supports --build and --host,
no --target in sight!

Any ideas?
Maxime.

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