--lib should produce reasonable output file names

[msullivan]

Right now, if you do "rustc --lib foo.rc", it will produce a library named "foo". It should produce a library named "libfoo.so" on linux and the equivalents on other platforms.

[brson]

The man page will need to be updated to reflect this fix.

[lht]

Only tested commit 06e5dad on Linux.

[brson]

I think we should take a different approach here, and resist more doing more filename munging in link_binary.

In rustc::driver::rustc::main there's already a big hunk of code dedicated to figuring out the correct name of the output file, and we have a function in std::os called dylib_filename that builds correct library names for the host platform. For now I think the best thing to do is put another check in main that uses dylib_filename when the compiler is building a library and isn't given an output file.

Ultimately all that filename manipulation in main should probably be separated from the call to compile_input and extracted to its own function.

[lht]

Thanks Brian for the comments and suggestions!

In commit 6b9d4b6, I tried to build the file names in rustc::driver::rustc::main.

Based on which, another commit 03f93aa, tries to do the "ultimate" thing. Lifts file name manipulation out of main altogether.

(Sorry, still only tested on Linux x64).

[brson]

This looks great, lht. Our main function is finally down to an appropriate size. I've merged both commits. Thanks.