Trouble when cross compiling rust-sasl in vendored mode

When cross compiling rust-sals in vendored mode, I ran into a few problems:

1. The tool chain was not automatically detected based on `HOST` and `TARGET`, although `CC_<target triplet>`, `CXX_<target triplet>` and `AR_<target triplet>` were configured.
2. After fixing 1. the cross compilation failed because `systest` was trying to run the cross compiled tests.