Use CMake FetchContent for curl

[dmehala]

Description

Rework curl dependency. Now, we use FetchContent which populates dependencies during the configuration stage and bringing all the targets declared by curl.

How does it fix DataDog/nginx-datadog#53?

The build system is now able to bring transitive dependencies into all targets that declare curl as a dependency.

Note to reviewer

I also added the equivalent of --without-zstd.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-03 17:40:42

Comparing candidate commit e9df5e1 in PR branch dmehala/curl-cmake with baseline commit 5c53143 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 0 unstable metrics.

[dgoffredo]

Nice!

The resulting .so, on my system, is larger now.

Before:

-rwxrwxr-x 1 david david 21943168 Dec  1 11:20 .build/libdd_trace_cpp.so

After:

-rwxrwxr-x 1 david david 23314640 Dec  1 11:18 .build/libdd_trace_cpp.so

I wonder if this is due to the CMake-flavored libcurl build including more code than the configure-based one.

As we've discussed before, the only concern I have with these changes is that they allow dependencies to creep into the build, unnecessary and unnoticed.

I think it's fine, though. This is good to merge.