Honor custom include directories in CMake exports

[fhgffy]

Problem

CMAKE_INSTALL_INCLUDEDIR controls where utf8proc.h is installed, but the exported utf8proc::utf8proc target always advertises <prefix>/include. Setting the directory to headers therefore produces an installation that the existing test/app consumer cannot use.

On current master (4bfe012), this reproduces with:

cmake -S . -B build -DCMAKE_INSTALL_INCLUDEDIR=headers \
  -DCMAKE_INSTALL_PREFIX="$PWD/prefix"
cmake --build build
cmake --install build
cmake -S test/app -B consumer -DCMAKE_PREFIX_PATH="$PWD/prefix"

The last command fails because the imported target references the nonexistent prefix/include directory. Both static and shared builds are affected.

Changes

• Initialize GNUInstallDirs before defining the target's include interface and use ${CMAKE_INSTALL_INCLUDEDIR} in INSTALL_INTERFACE
• Extend the existing CMake installed-consumer CI matrix with default include and custom headers layouts, keeping artifact names unique

Validation

Tested on Linux with GCC 14.2.0 and CMake 3.31.6:

• Reproduced the consumer failure on the unmodified base for static and shared custom-directory installs; default-directory controls pass
• Fixed static/shared × default/custom installs configure, build, link, and run, including after moving each installation prefix
• All 10 upstream CTests pass in each of the four configurations on both base and fixed trees (40 executions per tree), with Unicode 18.0.0 conformance data
• Nested relative paths, paths containing spaces, absolute include paths, and add_subdirectory with UTF8PROC_INSTALL=OFF pass
• Workflow YAML parsing and patch whitespace checks pass

Windows, macOS, MinGW, and the minimum supported CMake 3.10 were not run locally.