package/quota: fix static build with e2fsprogs

quota does not use pkg-config to find e2fsprogs libs, so it misses out
on the fact that it may require -pthread when doing static links.

Fix that by manually adding -pthread in case of the static build (of
course, only if the toolchain has threads).

Fixes:
    http://autobuild.buildroot.org/results/733/733145e94d7c4f3b11e867c60a1e8f8e049a3ce0/
    http://autobuild.buildroot.org/results/666/6666f7c07339830fb45a5decb8825560ee432ce5/
    http://autobuild.buildroot.org/results/f5b/f5b1d57bd26adaf125555b97ff1eb9faf223a293/


    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>