MINOR: build: add linux-glibc-legacy build TARGET

As discussed in issue #128, introduce a new build TARGET
linux-glibc-legacy to allow the build on old, legacy OS.

Should be backported to 2.0.

(cherry picked from commit cc1eb1619fbd09bed40c09e312099c1ccd79902c)
Signed-off-by: Willy Tarreau <w@1wt.eu>
2 files changed