GDB 8.3.1 gdbserver linker error: needs -lrt

Hi all;

I was trying to compile the latest GDB for my system and the link of
gdbserver failed:

.../lib64/libstdc++.a(chrono.o):function std::chrono::_V2::system_clock::now(): error: undefined reference to 'clock_gettime'
.../lib64/libstdc++.a(chrono.o):function std::chrono::_V2::steady_clock::now(): error: undefined reference to 'clock_gettime'

As you can see I compile with static libstdc++.  Also, I'm building
against a pretty old version of GNU/Linux with an older GNU libc
(CentOS 6.5 or so).

The problem is that on systems this old you need to add -lrt to the
link line in order to get clock_gettime(), and the configure script
doesn't look for this.

This causes both gdbserver and libinproctrace.so to fail to link,
unless I hack the makefiles.

Gdb itself links properly because I'm linking with lzma and those libs
happen to include -lrt:

  LIBLZMA = .../lib/liblzma.a -lrt


Oddly enough, adding GDBSERVER_LIBS="-ldl -lrt" to the top GDB make
command doesn't work (even for gdbserver: libinproctrace.so has no
equivalent customizable library). I didn't look through the build
system to figure out how my command line overrides are being lost but
that's also something that should be fixed...