Provide a tailored error message on Linux

I just ran into a case where I installed OpenSSL in a docker container but I
forgot to install pkg-config. Right now openssl-sys relies on pkg-config, so
print out a nice error about this.

1 files changed, 13 insertions, 0 deletions

diff --git a/openssl-sys/build.rs b/openssl-sys/build.rs
index a435b192..b6c8c2de 100644
--- a/openssl-sys/build.rs
+++ b/openssl-sys/build.rs
@@ -6,6 +6,7 @@ use std::ffi::OsString;
 use std::fs::File;
 use std::io::Read;
 use std::path::{Path, PathBuf};
+use std::process::Command;
 
 fn main() {
     let target = env::var("TARGET").unwrap();
@@ -100,6 +101,18 @@ Unfortunately though the compile cannot continue, so aborting.
         }
     }
 
+    if host.contains("unknown-linux") && target.contains("unknown-linux-gnu") {
+        if Command::new("pkg-config").output().is_err() {
+            msg.push_str(&format!("
+It looks like you're compiling on Linux and also targeting Linux. Currently this
+requires the `pkg-config` utility to find OpenSSL but unfortunately `pkg-config`
+could not be found. If you have OpenSSL installed you can likely fix this by
+installing `pkg-config`.
+
+"));
+        }
+    }
+
     if host.contains("windows") && target.contains("windows-gnu") {
         msg.push_str(&format!("
 It looks like you're compiling for MinGW but you may not have either OpenSSL or