Add OPENZL_USE_SYSTEM_ZSTD option to use system-installed zstd

[otegami]

Summary

Fixes #105

Add a new CMake option OPENZL_USE_SYSTEM_ZSTD that
allows using system-installed zstd library via
find_package(zstd CONFIG) instead of the bundled
version. This is useful for Linux distributions that
package zstd separately.

When enabled:

• Uses find_package(zstd CONFIG REQUIRED) to find system zstd
• Creates a libzstd alias target for compatibility with existing code
• Exported config includes find_dependency(zstd) for downstream projects

When disabled (default):

• Behavior unchanged (uses git submodule or FetchContent)

Type of Change

New feature (non-breaking change which adds functionality)

Test Plan

Build with system zstd

cmake \
	-S .\
	-B ../openzl.build \
	-DOPENZL_USE_SYSTEM_ZSTD=ON \
	-DOPENZL_BUILD_TESTS=ON \
	-DCMAKE_INSTALL_PREFIX=/tmp/local &&\
cmake --build ../openzl.build -j4 &&\
ctest --test-dir ../openzl.build --output-on-failure

Verify installation does not include zstd files

cmake --install ../openzl.build
ls /tmp/local/lib/ | grep libzstd # should be empty

Build with bundled zstd (default)

cmake \
	-S . \
	-B ../openzl.build \
	-DOPENZL_BUILD_TESTS=ON \
	-DCMAKE_INSTALL_PREFIX=/tmp/local &&\
cmake --build openzl.build -j4 &&\
ctest --test-dir ../openzl.build --output-on-failure

Verify installation includes zstd files

cmake --install ../openzl.build
ls /tmp/local/lib/ | grep libzstd
libzstd.a
libzstd.so
libzstd.so.1
libzstd.so.1.5.7

Test Configuration

• Compiler: GCC 13.3.0
• Build type: default
• Platform: Linux (Ubuntu)

[neiljohari]

Hi @terrelln, would you consider accepting this patch or something similar?

I'm building Rust bindings for OpenZL (vendored + custom FFI). My project also
depends on zstd-rs, which statically links its own zstd. Currently both libraries
compile zstd independently, resulting in duplicate symbols and ~10MB binary bloat.

With OPENZL_USE_SYSTEM_ZSTD, I could point OpenZL's CMake at the zstd that zstd-rs
already built, avoiding the duplication.