Install a software bill of materials

The license obligations of a Ceres build depend on the optional
dependencies it links against. Notices in the license file or in the
configuration output have been suggested to convey these obligations,
but such notices cannot be processed by license compliance tooling. A
software bill of materials in the standard SPDX format is the
established means for this purpose.

Install an SPDX SBOM describing the package, its license and the linked
libraries when CMake supports it. SBOM generation rejects generator
expressions in link libraries, so the Abseil linkage is resolved at
configure time and the SBOM is skipped if a static Abseil still requires
one.

Change-Id: Idf175a1d75f2cba4921461dd6efddc02e1b70b1f