BUILD: tools: properly guard __GLIBC__ with defined()

The test on the glibc versions based on #if (__GLIBC > 2 ...) fails to
build under -Wundef, let's prepend defined(__GLIBC__) first.

(cherry picked from commit 7b2108cad1aa963dae663bdabb3840132afbe6b5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed