nginx: fix broken proxy_pass feature

[NGINX ticket #748 - https://trac.nginx.org/nginx/ticket/748]

Accoriding to #748 or https://github.com/archlinuxarm/PKGBUILDs/issues/1161

,
respectively, proxy_pass is broken on some architectures without position
independent code.

The solution is adding -fPIE -pie as build flags.

Signed-off-by: Jens Rehsack <sno@netbsd.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>