Use strerror_r in safe_strerror if available

Use strerror_r in safe_strerror if available

Also stores the result in a thread-local static variable and
changes the return value to a const char*.

This is already important because Guile creates threads and
Python can create threads, but with the patch series here:
https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/176
GDB itself will create threads, too.

gdb/ChangeLog:

2019-10-31  Christian Biesinger  <cbiesinger@google.com>

	* configure: Regenerate.
	* configure.ac: Check for strerror_r.
	* gdbsupport/common-utils.h (safe_strerror): Change return value
	to const char * and document that this function is now threadsafe.
	* gdbsupport/posix-strerror.c (safe_strerror): Make buf
	thread_local and call strerror_r, if available.
	* utils.c (perror_string): Update.
	(print_sys_errmsg): Update.

Change-Id: I81048fbaf148035c221c528727f7efe58ba528eb