[cmake] Make pkg-config files relocatable

[tandede]

Problem

PCL's pkg-config templates expand CMAKE_INSTALL_PREFIX while the project is being configured. The resulting .pc files therefore retain the path used on the build machine.

This is especially visible in the Windows All-In-One installer: users may select a different installation directory, but the installed pkg-config files continue to reference the packager's original prefix. Moving an installed PCL tree has the same problem.

Approach

For a relative PKGCFG_INSTALL_DIR, derive the package prefix from ${pcfiledir} and the number of path components between the pkg-config directory and the installation root. This keeps the generated file independent of the build-time installation prefix.

Absolute pkg-config destinations retain the previous behavior because they are not necessarily located inside the installation tree.

The computed prefix is shared by both the regular and header-only pkg-config templates.

Changes

• Compute a relocatable PKGCONFIG_PREFIX in PCL_MAKE_PKGCONFIG.
• Use it in the regular and header-only .pc templates.
• Add a CMake regression test that checks every generated pcl_*.pc file against the expected prefix for the configured installation layout.

Results

With the default lib/pkgconfig layout, generated files now contain:

prefix=${pcfiledir}/../../

A deeper relative layout such as lib/custom/pkgconfig produces:

prefix=${pcfiledir}/../../../

Absolute pkg-config installation directories continue to use the configured CMAKE_INSTALL_PREFIX.

After installing into one directory and moving the complete installation tree, pkg-config --variable=prefix pcl_common and the reported include flags both resolve against the new location.

Validation

• Configured default, nested relative, and absolute pkg-config installation layouts.
• Ran the new prefix regression against all three layouts.
• Confirmed both regular and header-only generated files.
• Built the pcl_common target successfully.
• Installed and relocated the default layout, then queried its prefix and include flags with pkg-config.
• Ran git diff --check.

The complete global_tests configuration was not available locally because the environment does not contain the GoogleTest source package required by FindGTestSource. The new regression is a standalone CMake script and was executed directly for all three layouts.

Fixes #4240.