A previous change (#6445) was introduced to improve the relocatability of packages by using relative paths in the generated .pc files. However, this had an undesirable side effect when handling paths to external dependencies.

The Problem

For a package like libpng that depends on zlib, the paths to zlib were converted into deeply nested and fragile relative paths, which is not ideal.

For example, the Libs field was changed as follows:

- Libs:  -LC:/package/z/zlib/v1.3.1/95786712bac941e795afa187a68143a4/lib -L${libdir} -lpng -lzlib
+ Libs:  -L${libdir}/../../../../../z/zlib/v1.3.1/95786712bac941e795afa187a68143a4/lib -L${libdir} -lpng -lzlib

This path (${libdir}/../../...) is confusing and defeats the purpose of improving relocatability.

The Solution

This PR fixes the issue by applying a more precise rule for path conversion:

Paths within the package's own installation directory (installdir) are converted to be relative to ${prefix} or ${exec_prefix}.
Paths outside of the package's installdir (i.e., pointing to external dependencies) are kept as absolute paths.