gzip-vs-etag: Don't rename the ETag when gzip isn't ./configured

We should not rename the ETag header to work around the gzip module
clearing it, when the gzip module actually isn't built. In that case
we will fail to inject the header filter that renames the header back
to 'ETag' before it gets send out.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/770
As is this is kind of repetitive.  What if instead we did:

```
bool gzip_enabled = false;
#if (NGX_HTTP_GZIP)
gzip_enabled = true;
#endif
if (gzip_enabled &&
    StringCaseEqual(name_gs, "etag") &&
    StringCaseStartsWith(value_gs, "W/")) {...
```