The installed pkg-config meta-data is wrong when installing anywhere other than /opt/bb

When I build and install using:

```
eval $(bde_build_env.py --cpp-std=17 --build-type=Release --build-dir=/tmp/bde --install-dir=/home/my/bde)
cmake_build.py configure
cmake_build.py build
cmake_build.py --install_dir=/home/my/bde --prefix=/ install
```

The headers and libraries are correctly installed to `/home/my/bde` but the installed pkg-config meta-data, e.g. `/home/my/bde/lib/pkg-config/bdl.pc` contains:

```
prefix=/opt/bb
libdir=${prefix}/lib
includedir=${prefix}/include

Name: bdl
Description: The bdl package.
Version: 3.109.0
Requires:
Requires.private: bsl inteldfp pcre2
Cflags: -I${includedir}
Libs: -L${libdir} -lbdl
```

That is the prefix is `/opt/bb` even though that's not what I specified and that's not where the files have been installed.