Prefer MariaDB Connector/C for Spine builds

[somethingwithproof]

Summary

• prefer mariadb_config while retaining mysql_config support
• validate the MySQL-compatible header and client symbol instead of probing library filenames
• retain MariaDB, MySQL, and Percona-compatible fallback linking
• exercise both MariaDB Connector/C and MySQL Connector/C with GCC and Clang in CI
• document MariaDB Connector/C as the preferred packaging dependency

Validation

• Docker: full make check with MariaDB Connector/C (4/4 passed)
• Docker: full make check with MySQL Connector/C (4/4 passed)
• Docker: build through header/library fallback with the config tool unavailable
• actionlint .github/workflows/ci.yml
• git diff --check
• GitHub Actions: all nine jobs succeeded, including all four compiler/connector combinations

Coordination

PR #597 overlaps configure.ac and CI structurally but retains the prior connector discovery. It should rebase after this PR and preserve these checks; no implementation is duplicated here.

Closes #612

[Copilot]

🟡 Changes recommended

The no-config-tool fallback can detect headers under non-default prefixes but fail to add corresponding library search paths, causing avoidable link failures in valid installations.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates Spine’s build and CI configuration to prefer MariaDB Connector/C while retaining compatibility with MySQL/Percona client libraries, and refreshes documentation to reflect the preferred dependency.

Changes:

• Reworks configure.ac to prefer mariadb_config (with mysql_config support) and validates availability via mysql.h + mysql_init rather than probing library filenames.
• Expands CI to build/test across a compiler (gcc/clang) × client (MariaDB/MySQL Connector/C) matrix.
• Updates installation docs and changelog to note MariaDB Connector/C as the preferred packaging dependency.

File summaries

File	Description
README.md	Documents MariaDB Connector/C dev package as preferred over MySQL Connector/C.
INSTALL	Mirrors the README dependency guidance for Cygwin prerequisites.
configure.ac	Switches client discovery to config-tool flags when available; otherwise uses header + link checks.
CHANGELOG	Adds an entry tracking the feature/issue work for connector discovery changes.
.github/workflows/ci.yml	Adds CI build matrix for gcc/clang and MariaDB/MySQL client dev packages.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Lite

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[somethingwithproof]

Consolidated into #618 to reduce review overhead. #618 now carries the connector discovery, compiler/connector matrix, shared build action, sanitizer test execution, and Linux-capabilities CI coverage. No implementation was dropped.

[somethingwithproof]

Superseded by #618.