gdal.pc contains wrong paths if CMAKE_INSTALL_<dir> is absolute

[dotlambda]

Expected behavior and actual behavior.

I'm building gdal on NixOS and obtaining the following in lib/pkgconfig/gdal.pc:

CONFIG_VERSION=3.5.0
CONFIG_INST_PREFIX=/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3
CONFIG_INST_LIBS=-L/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3//nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3/lib -lgdal
CONFIG_INST_CFLAGS=-I/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3//nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3/include
CONFIG_INST_DATA=/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3/share/gdal
prefix=${CONFIG_INST_PREFIX}
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${exec_prefix}/include
datadir=${CONFIG_INST_DATA}

Name: libgdal
Description: Geospatial Data Abstraction Library
Version: ${CONFIG_VERSION}
Libs: ${CONFIG_INST_LIBS}
Libs.private: -lcryptopp -lssl -lcrypto -llzma -ldeflate -llz4 -larmadillo -lqhull_r -lgeotiff -ltiff -lheif -lLerc -lzstd -lbrunslienc-c -lbrunslidec-c -ljpeg -lpoppler -lpng -lgif -lnetcdf -lblosc -lcfitsio -lhdf5 -lcurl -lwebp -lhdf -lmfhdf -lz -lopenjp2 -L/nix/store/iyf9g7w9r448wimavwwbdjw320wlrjc4-tiledb-2.3.3/lib64 -ltiledb -lIlmImf -lIlmImfUtil -lHalf -lIex -lpcre2-8 -lspatialite -lsqlite3 -lxerces-c -L/nix/store/fmbxbsil9y1dvgl0ph7llirdz7mx29f0-mariadb-connector-c-3.2.5/lib/mysql -lmysqlclient -lpq -lparquet -larrow -ldl -lexpat -lxml2 -lgeos_c -lproj -ljson-c -lstdc++ -lm
Cflags: ${CONFIG_INST_CFLAGS}

I would expect

CONFIG_INST_LIBS=-L/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3/lib -lgdal
CONFIG_INST_CFLAGS=-I/nix/store/crsca8wbc9c0g3kmjcpc6b7pgxxygkz7-gdal-3.5.0.3/include

See also https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path.
This was not an issue when using autotools with GDAL 3.4.

Steps to reproduce the problem.

Run nix-build -A gdal on NixOS/nixpkgs#179406.

Operating system

NixOS 22.11-pre

GDAL version and provenance

3.5.0.3 from GitHub tarball

[Artturin]

We also have a large pr fixing other packages so you can use that as a reference NixOS/nixpkgs#172150 when fixing this issue