Protobuf configure may need more work

[robe]

As noted in #3661, at least building under mingw, it was not sufficient to specify the --with-protobufdir directory path.

I tried all sorts of permutations to figure out what's required if you are installing in non-standard place.

It seems I had to do all the following:

1) Specify the --with-protobufdir (without this if I had protobuf and protobufc in my path, it would pass configure, but fail at compilation with can't find include. Though I realized now if I include include in my path that might have done the trick.

2) Configure failed at finding PROTOBUFC, unless I also

had:

export PKG_CONFIG_PATH=path/to/protobuf/lib/pkgconfig

3) Tried as I did, I couldn't get it to work if I had protobuf and protobufc installed in separate directories.

This could all be user error, but at lease probably should be documented so people needing to build from source and not wanting to install in default locations (e.g. cause they are packagers) don't run thru all this nonsense.

[robe]

I think this was probably address with Bjorn's commit detailed here:

​https://github.com/postgis/postgis/pull/123

So I'm just gonna close this out as done.