impl OPENSSL_SRC_PREFIX_SANTIZE env var

[ShootingKing-AM]

• Sanitize path forcefully based on env var
  • 2 use cases as issues Error while cross compiling from Windows 10 to Raspberry Pi 2 (armv7-unknown-linux-gnueabihf) #118
• Dump some helpful build info when build fails

[alexcrichton]

Thanks for this PR, but is there a need for this to be an env var? Can this be automatically detected when this sanitization on Windows is required?

[ShootingKing-AM]

Actually I also thought so, but i dont know all kinds of cases people use this crate in. So thought I wouldn't want to make any breaking changes.

Otherwise, we can enforce some consistency with --openssldir='s path's os type (i guess)