xdt-autogen: Various small improvements

- quoting
- replace backticks with $()
- replace 'test "x$var"' with 'test "$var"'
- replace $(pwd) with $PWD
- simplify the command to search for languages
- do not use subshell when unneeded
- do not use redirection when unneeded
- directly test command exit codes instead of using $?
- readability (useless ";", indenting)