Upgrade GDAL to 3.1.0

[nickpeihl]

GDAL 3.1.0 is now available. this pr required a few other changes aside from the url.

• --with-mandir is no longer supported as a configure option. so I removed what I believe are the relevant code parts.
• --with-curl was changed to use the curl-openssl dependency location. but I'm not sure why this was needed.
• --with-xml2 only acceptsyes/no. GDAL now uses pkginfo to determine the path for libxml2.

I ran brew audit --strict osgeo-gdal but it returned a lot of issues that seem out of scope for this PR as they already exist onmaster`.

please let me know if you have questions or concerns.

[alazarolop]

Hi @nickpeihl thank you for the PR. The main concern with a GDAL upgrade is compatibility issues with dependencies. Master branch is aimed for complete releases. Could you apply the PR to bottles branch instead? So we can test it there.

Apart from that, it would be interesting to deal with the other issues in a different PR. Thank you

[nickpeihl]

Closing this PR. See #1379 instead.