TARGET perfix for envs.

[xuxiaocheng0201]

Current env is hard-coding, like OPENSSL_SRC_PERL.
But in crate openssl, these variables can be prefixed with the upper-cased target architecture, doc.
I hope this crate can read envs with target perfix, like read aarch64_linux_android_OPENSSL_SRC_PERL. This will be very useful when cross compiling.

[alexcrichton]

I think this might actually be the only env var for this crate, or is there another one you're interested in? For this one specifically I'd naively think that there's not much benefit from having a target-prefix because it's only used as a build tool which must run on the host anyway. Or is there a different problem you're trying to fix?

[xuxiaocheng0201]

Yeah, it's the only env I wanted.

Previously I used cygwin to compile openssl with vendored feature. And the perl path is different from the host windows. So I opened this issue.

But I failed, and am using rustls now. This issus is not necessary for me.