[fhgffy]

With absolute CMAKE_INSTALL_LIBDIR or CMAKE_INSTALL_INCLUDEDIR, CMake installs tinyxml2 to the requested location, but tinyxml2.pc prepends the install prefix again. A downstream pkg-config consumer then fails to find the installed header or library.

Keep absolute install directories unchanged and retain ${prefix} / ${exec_prefix} for relative directories. Add a metadata regression test and an absolute-directory configuration to the existing CI matrix.

Validated on master 8224e427b655b83dae5e2298f1e6919523a78737, Linux x86_64, GCC 14.2.0, CMake 3.31.6 and pkg-config 1.8.1:

• The regression fails on the original source for absolute include, absolute lib and both-absolute layouts. Default and custom relative layouts pass.
• Fixed pkg-config consumers compile, link and run in all five layouts for static/shared Release builds, plus both-absolute static/shared Debug builds.
• Original and fixed upstream tests and installed find_package tests pass in static/shared Debug/Release. Each xmltest run reports 528 passed, zero failed.
• Independent review additionally checked compiler wrappers, staged installs with DESTDIR / PKG_CONFIG_SYSROOT_DIR, and prefix overrides.

Windows, macOS and CMake 3.15 were not run locally. The existing CI matrix covers those environments; remote CI has not run for this patch.