[build, lto] Only accept -fuse-linker-plugin if linker supports -plugin (PR lto/46944)

[Rainer Orth]

Hi Ralf,

>> I haven't found if there are provisions for in-tree gold, though, and
>> still cannot test that.
>
> I'm not quite sure I understand this statement.  I built a combined tree
> with gold enabled a while ago (must've been several months now).
> I might be misunderstanding this.

I suppose I've been unclear: I'm specificially referring to the current
code for setting gcc_cv_lto_plugin: in the in-tree case, there's nothing
that deals with in-tree gold.

>>    if test $in_tree_ld = yes -a x"$ORIGINAL_PLUGIN_LD_FOR_TARGET" = x"$gcc_cv_ld"; then
>> -    if test "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -ge 21 -o "$gcc_cv_gld_major_version" -gt 2; then
>> -      gcc_cv_lto_plugin=2
>> -    elif test "$ld_is_gold" = yes -a "$gcc_cv_gld_major_version" -eq 2 -a "$gcc_cv_gld_minor_version" -eq 20; then
>> -      gcc_cv_lto_plugin=1
>> -
>> +    ld_ver="GNU ld"
>> +    # FIXME: ld_is_gold?
>
> What about this FIXME?  Did you test gold?  Is it not relevant here?
> Can the FIXME go?

I cannot test gold since it doesn't yet work on Solaris: cf. binutils PR
gold/12525.  We made some progress on that front, but the PR is
currently stalled and I had other things on my plate that prevented me
from pushing it.  As I said, the current code (before my patch) doesn't
handle in-tree gold, so I don't feel obliged to change that, especially
since I'm in no good position to test.

>> +      ld_vers_major=`expr "$ld_vers" : '\([0-9]*\)'`
>> +      ld_vers_minor=`expr "$ld_vers" : '[0-9]*\.\([0-9]*\)'`
>
> Can you try the expr statements quickly on Tru64?  If not, I can do it
> for you ('info Autoconf --index expr' is long and tells of many woes).

I just tried with /bin/expr and ld_vers set to 2.20.1.  OTOH, this isn't
relevant for two reasons: this code is identical to the one determining
ld_vers_major/ld_vers_minor further up in gcc/configure.ac, and GNU ld
(as well as GNU as) aren't currently supported on Tru64 UNIX and I
seriously doubt that will change over the remaining livetime of the
platform.

> Thanks, and sorry for the delay,

No worries.  I'd just like to get this series of patches out of my queue
(and eventually backported to the 4.6 branch if all issues are sorted
out).  Maybe one of the build maintainers finds some time to handle the
current mess that are the linker tests in gcc/configure.ac, compared to
what we do for the assembler.

Thanks.
	Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University