option to disable libbrotli in Makefile

[LeGEC]

Database name

all databases - affects using libbrotli when building wal-g

Issue description

In its current state, the Makefile on master does not allow to build a pure go version of wal-g : it always builds and link libbrotli.

Describe your problem

We have a project that uses pure go binaries, and wanted to keep it that way once we chose to use wal-g. We thus compile our own version of wal-g, where we disabled brotli and chose a pure go implementation of zstd.

I suggest to add an option to completely disable libbrotli in the regular Makefile.

The downside of using that option at compile time is that it creates wal-g binaries which can't handle brotli anymore (especially: that wouldn't be able to read back previous files compressed with brotli).
This could be addressed by using a pure go broti implementation -- https://github.com/andybalholm/brotli claims to do that but we haven't taken time to review it.

In our case, we are fine with disabling brotli compression/decompression entirely (as said above: we use zstd, and use a pure go library that implements it), but obviously projects that used brotli for previous backups should keep using versions of wal-g that support it.