building GDB 9.0.50.20191206

[Tom Tromey]

Tom> Review here:
Tom>     https://sourceware.org/ml/gdb-patches/2019-08/msg00433.html
Tom> Maybe some comments there still need to be addressed.  I can't recall
Tom> offhand.

I've updated my patch and re-read the thread.

The remaining issue is that I still haven't decided what to do about
<config.h> in files like nat/linux-btrace.c.  The message referred to
above outlines the various approaches and their problems pretty well.

Tom