nginx-gridfs: Add configure option to build with wno-error

Some modules add things to CFLAGS that will make ngx_pagespeed emit
warnings at compile time. For example, nginx-gridfs will add
`--std=c99` - which is no good for ngx_pagespeed.

To work around that, make configure add `-wno-error` when used like
this: `WNO_ERROR=YES ./configure`
On my system, that results in a succesfull build when nginx-gridfs
is added to the module mix.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/626