Use full path to packages specified

CMake will change to the source directory when processing the
CMakeLists.txt file which causes relative paths to be based on this
instead of at the build directory which is probably assumed by the
user.

configure

@@ -78,6 +78,8 @@ while getopts -- ":-:" optchar; do
           pkgname=${pkgname#with-}
           # get the location of the package; everyhing after equal sign
           pkgloc=${OPTARG#*=}
+          # expand to full path since CMake changes to source directory (!)
+          pkgloc=$(echo $(sh -c "cd $(dirname ${pkgloc}); pwd")/$(basename ${pkgloc}))
           # packages need different suffix for their root (sic)
           case "${pkgname}" in
             agmg  |\