Fix pkg-config paths for absolute CMake install directories

[fhgffy]

Summary

Handle absolute CMake install directories in libtommath.pc while preserving the existing prefix-relative form for relative directories.

With BUILD_SHARED_LIBS=ON and an absolute CMAKE_INSTALL_INCLUDEDIR or CMAKE_INSTALL_LIBDIR, installation already places files in the requested directory. The current pkg-config template nevertheless prepends ${prefix}/, producing include/library paths that do not exist. An installed consumer fails to find tommath.h, or fails to link -ltommath when only the library directory is absolute.

The fix computes each pkg-config directory independently. The shared Makefile also uses this template, so its substitutions are updated to preserve its existing byte-for-byte output.

Tests

• A new shared-build CTest covers relative, absolute, and both mixed directory layouts without performing an installation
• That regression fails on unchanged production code and passes with the fix
• Separate installed pkg-config consumers compile, link and run for all four layouts plus custom relative directories
• A relocated relative installation works with pkg-config's prefix override
• Existing CMake tests pass in both static and shared modes, as does helper.pl --check-all
• The actual makefile.shared metadata recipe produces identical output before and after the patch for default and custom prefixes

Tested on Linux x86-64 with GCC 14.2.0, CMake 3.31.6 and pkg-config 1.8.1. Full libtool builds and other platforms were not run.