meson: fix dirname/basename confusion in meson-and-wants.sh install helper (#10126)

meson: fix dirname/basename confusion in meson-and-wants.sh install helper (#10126)

We would create a useless empty directory under build/.
It seems we were lucky and all symlinks were installed into directories
which were alredy created because we installed something into the same
location earlier.

While at it, also add '-v' to 'mkdir -p'. This will print the names of
directories as they are created (just once), making it easier to see all of
what the install script is doing.