CMake: Absolute package configuration install path

[ethanmusser]

Description

The package configuration install path is absolute. While configuring with CMAKE_INSTALL_PREFIX works, this means that the more-current CMake install syntax does not work. E.g., the following command will install headers, libraries, and exported CMake targets to the install/ directory, but the package configuration files will end up in /usr/local.

cmake --install build/ --prefix install/

Steps to reproduce

1. Configure without setting CMAKE_INSTALL_PREFIX and build.

   cmake -S . -B build && cmake --build build

2. Install using the newer syntax.

   cmake --install build/ --prefix install

3. Examine the contents of install/ (which will contain the headers, libraries, and exported targets) and /usr/local (which will contain the package configuration files).

Expected behavior

All installed artifacts should end up under the specified install prefix.

Environment

OS: Ubuntu 24.04
CMake: 3.28.3

Additional information