Add config file for easy building

This is to be used in conjunction with --add-module at nginx
configure time.

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>

config

+ngx_addon_name=ngx_http_shibboleth_module
+HTTP_MODULES="$HTTP_MODULES ngx_http_shibboleth_module"
+NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_addon_dir/ngx_http_shibboleth_module.c"