This defines _FORTIFY_SOURCE in common-defs.h. This seems like a
sensible safety measure, and also it may help avoid build problems
with -Wunused-result on distros that already define _FORTIFY_SOURCE by
default.
Tested by the buildbot.
gdb/ChangeLog
2018-08-13 Tom Tromey <tom@tromey.com>
* common/common-defs.h (_FORTIFY_SOURCE): Define.