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.

@peterbowey mentioned that -wno-error fixes the build -- so
to work around, 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

config

@@ -111,6 +111,10 @@ case "$NGX_GCC_VER" in
   ;;
 esac
 
+if [ "$WNO_ERROR" = "YES" ]; then
+    CFLAGS="$CFLAGS -Wno-error"
+fi
+
 pagespeed_include="\
   $mod_pagespeed_dir \
   $mod_pagespeed_dir/third_party/chromium/src \