Meta: Convert build directory path to be completely absolute

[Jan200101]

Differences in path stem causes pkgconf to ignore the sysroot completely.

Meson in particular, others build system might do this too, messes with PKG_CONFIG_LIBDIR causing the stem to differ and pkgconf to believe any pc file it finds is outside the sysroot.

This has been fixed in pkgconf since at least pkgconf 1.9.0 (this commit to be specific), but distros have been painfully slow in moving forwards