PATCH: Configure libstdc++-v3 against gthr.h

[Loren James Rittle]

All remaining thread configuration patches for C++ have been applied
to mainline.  Minor tweaks were applied based on feedback:

Phil: #error lines were merged
Dave: #warning lines were removed (with the expectation that further
      work will be done to address the reason for the warning - I am
      working on this now).

I am told that I have authority to put them on the 3.0 branch already.
And, I have been asked to do so ASAP.  How about this compromise
position:

I need at least one person to update from the mainline for your
untested primary evaluation host (i.e. mips-sgi-irix6.5
powerpc-ibm-aix4.3.3.0 or sparc-sun-solaris2.7); then bootstrap it
however it has been building (if you can do only one) and
--enable-threads or --disable-threads (whichever is the opposite of
how you usually build, if you can do both at once).

I have access to sparc-sun-solaris2.7 and it does bootstrap
completely, but so far it doesn't actually work with or without my
patches... (Yikes, that is a primary platform, no?  I am studying the
current install.texi and will upgrade binutils and give it another go
tonight.)

Once those results are reported and any followup patching has occurred
as required on mainline, I will move the patch to the 3.0 branch.

If asked to do so without the above condition having been met come
Friday afternoon (CST), then I will do so anyways.

Regards,
Loren