add c makefile

[samansmink]

Makefile for new C/C++ extension template targeting.

This is a similar template to https://github.com/duckdb/extension-template-rs, but then for C/C++ based extensions. Note that this is different from the current standard template https://github.com/duckdb/extension-template/actions which is also intended for C++ extensions but uses DuckDB's C++ API and reuses its build system and test infrastructure.

edit: added a small tweak because things were still broken

[carlopi]

Thanks! Looks good!

I would consider changing the naming structure to be like:

makefiles/
  ... duckdb_extension.make
  ... duckdb_capi_extension_c.make
  ... duckdb_capi_extension_rs.make
  ... duckdb_capi_extension.make

or something along those lines, where it's clear that duckdb_capi_extensions (whatever the naming schema) are different than duckdb_extension.make

I think we are still in time (before adding more repos) to change this with little cost.