Panics when building a #![no_std] crate on linux

[jahartsell]

It seems like some crates can have an empty link line, which causes a panic when determining the static libs here

cargo-c/src/build.rs

Line 1158 in f873c83

exec.link_line

I assume static libs should just be empty in that case, and the following at least builds my case.

diff --git a/src/build.rs b/src/build.rs
index 273efc1..59b673c 100644
--- a/src/build.rs
+++ b/src/build.rs
@@ -1160,7 +1160,7 @@ pub fn cbuild(
                     .unwrap()
                     .values()
                     .next()
-                    .unwrap()
+                    .map_or("", |s| s)
                     .to_string()
             };
             let capi_config = &cpkg.capi_config;

The following example reproduces the issue for me using rust 1.80.1 and cargo-c 0.10.3+cargo-0.81.0 on Ubuntu 22.04

Cargo.toml

[package]
name = "testlib"

[features]
capi = []

[package.metadata.capi.library]
rustflags = "-Cpanic=abort"

lib.rs

#![no_std]

#[cfg(feature="capi")]
mod capi {
    #[panic_handler]
    fn panic_handler(_info: &core::panic::PanicInfo) -> ! {
        loop {}
    }

    #[no_mangle]
    extern "C" fn example() -> u32 {
        0
    }
}

[lu-zero]

If you can send a PR I'd land it before making the next release, sorry for the belated reply but sometimes github messages get lost in the mailbox :/