sendmail: link with gcc instead of ld

sendmail: link with gcc instead of ld

If ld is used directly to link libmilter.so, linking is not done
correctly with glibc, and produces a library that can't be used.
Linking with the bad library produces a strange error:
ld: conftest: hidden symbol `stat' in [...]libc_nonshared.a(stat.oS)
is referenced by DSO

Using ld with musl works fine, but using gcc is a more portable way of
linking the shared library anyway.

Signed-off-by: Eneas U de Queiroz <[email protected]>