pc file includes absolute paths

Hi,

I've noticed that the .pc file generated by the pkg-config call in libssh2-sys's build.rs includes absolute paths to the build directory. This causes issues for build systems that check the build output for reproducibility (and my particular case leads to cache misses for prebuilt artifacts).

Would things break if the paths were relative or if the .pc file simply wasn't generated? I'm using this as a workaround for now:

diff --git a/build.rs b/build.rs
index c4425ee..f99f132 100644
--- a/build.rs
+++ b/build.rs
@@ -173,6 +173,7 @@ fn main() {
         .unwrap();
     let version = &version_line[version_line.find('"').unwrap() + 1..version_line.len() - 1];
 
+    /*
     let pkgconfig = dst.join("lib/pkgconfig");
     fs::create_dir_all(&pkgconfig).unwrap();
     fs::write(
@@ -188,6 +189,7 @@ fn main() {
             .replace("@LIBSSH2VER@", version),
     )
     .unwrap();
+    */
 
     cfg.warnings(false);
     cfg.compile("ssh2");

Start in libssh2-sys/build.rs around the pkg-config call at line 179 and inspect how the generated .pc file receives build-directory paths. Check whether relative paths or omitting the .pc file preserves the expected build output. Done means the generated output no longer embeds absolute build paths without breaking the reported libssh2 metadata.