building project with folly yields "/usr/bin/ld: cannot find -lgflags_shared"

Hi,
I used to be able to use Folly, but something broke. I'm using Arch Linux, maybe there's a change in how they packaged it?

This is all I have in /usr/lib from glfags; no libgflags_shared.

```gflags /usr/lib/libgflags.a
gflags /usr/lib/libgflags.so
gflags /usr/lib/libgflags.so.2.2
gflags /usr/lib/libgflags.so.2.2.2
gflags /usr/lib/libgflags_nothreads.a
gflags /usr/lib/libgflags_nothreads.so
gflags /usr/lib/libgflags_nothreads.so.2.2
gflags /usr/lib/libgflags_nothreads.so.2.2.2
```
Is there a quick obvious solution here? Thanks.