Avoid empty include paths (i.e. cc -I "" ) as they are not supported by GCC. Fix #311

openssl-sys/build.rs

@@ -15,8 +15,11 @@ fn main() {
         // rustc doesn't seem to work with pkg-config's output in mingw64
         if !target.contains("windows") {
             if let Ok(info) = pkg_config::find_library("openssl") {
+                // avoid empty include paths as they are not supported by GCC
+                if info.include_paths.len() > 0 {
                     let paths = env::join_paths(info.include_paths).unwrap();
                     println!("cargo:include={}", paths.to_str().unwrap());
+                }
                 return;
             }
         }