Fix PATH separator handling

which is different on Windows. Need to check if the same separator is used for CAML_LD_LIBRARY_PATH and such though,
for now we assume the same separator is used for all variables
Add variables to query the available ocaml tools: ocaml-native, ocaml-tools-opt

Closes #979 and #672
Merge pull request #1138 from AltGr/ocaml-opt-available

Added variables `ocaml-tools-opt` and `ocaml-native`