Modify cmExportPackageInfoGenerator to record not just what packages are
required, but what targets (components) are used by those requirements,
and to populate the requirements' component lists accordingly.
In commit 3d52d70b84 (export: Add initial CPS support, 2024-07-26) we
added a static definition of a `constexpr` member to help older
AppleClang and SunPro linkers. This is deprecated by C++17 and Clang
now warns about it, so preserving the approach would require complex
conditions. There is no significant benefit to `constexpr` here, so use
a traditional file-scoped definition instead.
Add initial support for exporting (install only, for now) Common Package
Specification (https://cps-org.github.io/cps/) format package
descriptions. This has some limitations, such as not supporting
generator expressions (as these cannot be portably exported), and only
partially supporting transitive dependencies, but should be usable for
at least some simple cases. (Actually, $<LINK_ONLY> is theoretically
supportable, but is not yet implemented.)
This still needs tests; these will be added in the next commit. Other
potential improvements include support for language-specific compile
definitions and inferring some package properties from project
properties. Additionally, there is no module support yet; this is partly
pending on having a tool agnostic format for providing the necessary
information.
