[meson] need -lrt with glibc < 2.17 (fixes #3120)
[meson] need -lrt with glibc < 2.17 (fixes #3120)

(thx gzpapp at gmail dot com)

x-ref:
  "Building on glibc 2.16 where clock_gettime() is in librt.so"
  https://redmine.lighttpd.net/issues/3120

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4