Properly escape paths to executables. Fixes #18632.

[vharavy]

1. Introduce putpathvar function that prints variable shell-quoted by using %q format specifier. This function is used within probe to save the result into config.tmp.
2. Removes search-and-replace pattern that transforms \ into / as it messes up shell-quoted strings.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

[aturon]

cc @alexcrichton @brson

[brson]

Thanks @vharavy! 🍰

[vharavy]

You are welcome! But it seems there is a problem. Shell from busybox does not recognize %q format qualifier. I will try to find a more portable way to do it.

[vharavy]

Okay. Instead of using %q with printf I put all paths in double quotes instead. make passes successfully on Windows 7 64-bit and Arch Linux. Also, I corrected the way the version of Pandoc is obtained to be compatible with how Pandoc reports its version on Windows.

[vharavy]

I have updated configure script to prevent it from putting double quotes around empty variables as it messes up make's conditional:

ifeq ($(CFG_PANDOC),)
     ...