configure: fix some issues with last commit

 - after piping with | tr -d, an exit code was lost
 - suspicious use of " " inside a larger " ", we use ' ' now instead
configure: CAML_LD_LIBRARY_PATH is enriched, not overwritten

 Keeping the earlier content of this variable is crucial for opam
 (at least).

 Thanks to François Bobot and Thomas Refis for this one...