[PATCH, libstdc++] Fix missing gthr-default.h issue on libstdc++ configure

[Olivier Hainque]

Hello,

The change apparently broke targets that have their gthr- header not
in libgcc/ directly, quite a few.

We are seeing failure on ppc-aix for example, which has 

  ./libgcc/config/rs6000/gthr-aix.h

We are also seeing failures while building a win32 canadian
from linux, where we have

  ./libgcc/config/i386/gthr-win32.h

This one is really problematic.

The failures we are seeing are all of the form:

<<
make[5]: Entering directory `<bla>/obj/powerpc-ibm-aix5.2.0.0/libstdc++-v3/include'
make[5]: *** No rule to make target `<bla>/src/libstdc++-v3/../libgcc/gthr-aix.h', needed by `powerpc-ibm-aix5.2.0.0/bits/gthr-default.h'.  Stop.
>>

This, in particular:

+  glibcxx_thread_h=gthr-$target_thread_file.h

-${host_builddir}/gthr-default.h: ${toplevel_builddir}/libgcc/gthr-default.h \
+${host_builddir}/gthr-default.h: ${toplevel_srcdir}/libgcc/${glibcxx_thread_h} \

looks very suspicious wrt gthr headers buried in libgcc
target subdirs.

Can you please have a look ?

Thanks in advance,

Olivier