Fix cross-compile by adding LD variable

[bkuhls]

Partly reverts commit a449ff8.

Hard-coding "ld" when cross-compiling is a bad idea.

[bkuhls]

Issue was found by buildroot checks, all target archs fails except host arch x86_64:
https://gitlab.com/jolivain/buildroot/-/pipelines/2134120759
For example aarch64:
https://gitlab.com/jolivain/buildroot/-/jobs/11936875772

/builds/jolivain/buildroot/br-test-pkg/arm-aarch64/host/bin/aarch64-none-linux-gnu-gcc dkim.c dkim_transport.c pdkim.c signing.c
[...]
ld: unknown architecture of input file `dkim_tmp.o' is incompatible with i386:x86-64 output
ld: unknown architecture of input file `dkim_transport.o' is incompatible with i386:x86-64 output
ld: unknown architecture of input file `pdkim.o' is incompatible with i386:x86-64 output
ld: unknown architecture of input file `signing.o' is incompatible with i386:x86-64 output

[bkuhls]

Bug filed: https://bugs.exim.org/show_bug.cgi?id=3170