Fix: force the use of our _FORTIFY_SOURCE definition

Some toolset (ubuntu) already defined the _FORTIFY_SOURCE.

This removes the warning we see if this is the case. Unset the variable and
reset it.

The warning:

make[3]: Entering directory '/home/joraj/lttng/master/lttng-tools/tests/regression/kernel'
  CC       select_poll_epoll-select_poll_epoll.o
<command-line>: warning: "_FORTIFY_SOURCE" redefined
<built-in>: note: this is the location of the previous definition
  CCLD     select_poll_epoll

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I2fa0482afa8941705a992f62a2e63657ea9e2b87
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>