Export an installed CMake package with consistent ABI metadata

[dimpase]

The current CMake build has SONAMEs, GNUInstallDirs, TBB selection, and pkg-config installation, but lacks a consumable CMake package. It also reconstructs mathic and memtailor as static imported libraries without their usage requirements. The CMake-generated pkg-config file omits MATHICGB_DEBUG, and a direct configuration-dependent export would select the consumer's Debug ABI instead of the installed library's ABI.

Install mathicgbConfig.cmake, version metadata, and mathicgb::mathicgb, with C++17 and transitive dependencies. Prefer dependency CMake configs, then pkg-config metadata, and retain header/library discovery as a fallback for older installations such as the current Homebrew bottles. Preserve the current TBB ON/OFF/auto behavior and SONAME setup.

Export the installed library's Debug setting through both metadata formats, retain the TBB/no-TBB usage requirements, and explicitly install static archives and the public header. Install documentation using CMAKE_INSTALL_DOCDIR instead of the nonstandard licenses/mathicgb directory, allowing Gentoo to select /usr/share/doc/${PF}. Enable root-level CTest. Extend the existing Linux/macOS, Debug/Release, TBB-on/off CI matrix with shared/static installation checks, pkg-config flag checks, and separate Release consumers of relocated installations. Add the templates and fixtures to Autotools distributions and document usage. CMake 3.18 is required for aliases of non-global imported dependency targets.

macOS CMake CI uses the existing Homebrew memtailor and mathic packages directly. The fallback allows this even when their bottles lack CMake config and pkg-config metadata. Installed pkg-config flag checks run when the dependencies provide .pc files; the installed CMake consumer is tested on both platforms.

Local Gentoo validation, at low priority with two compilation jobs:

• All 248 tests passed for Release/shared with TBB and Debug/static without TBB.
• Relocated installed CMake consumers and pkg-config consumers passed, including Release consumers of Debug libraries.
• Dependency CMake configs were exercised in Release; pkg-config discovery of both dependencies was forced in Debug.
• make dist succeeded with a nonstandard mathic prefix and includes the new CMake config and consumer fixtures.
• Workflow lint and diff checks passed. Before the documentation-path follow-up, the full hosted Linux/macOS CI matrix passed.

The documentation-path follow-up was checked by applying the updated Gentoo patch to the release and staging an installation: documentation is under /usr/share/doc/mathicgb-1.4-r2, with no /usr/licenses. Overlay QA passed.

The discovery fallback was tested with both dependency CMake configs and pkg-config disabled. Mathicgb and mgb built against installed headers and libraries; a staged installed CMake consumer compiled, linked, and passed its runtime test.

Related work: Macaulay2/memtailor#23 and Macaulay2/mathic#29. This adapts the Gentoo packaging work to current mathicgb master without reverting its newer CMake improvements. External mathicgb selection in M2 remains separate.

[dimpase]

OK, looks good. Can be simplified once memtailor and mathic cmake changes are propagated into new releases (and into upstreams)

@d-torrance - over to you

[d-torrance]

I think we should keep these as a fallback. In particular, this would mean we could keep using the homebrew packages in CI instead of having to build them from source.

Same comment for mathic.

[dimpase]

as we maintain the Homebrew taps in question, we can just update them, then everything will be OK. Anyhow, you merged my mathic PR already...

[dimpase]

anyhow, fixed by afacdb8

please see Macaulay2/mathic#30 for the corresponding fix for mathic.