Set link type based on --lib flag

cli/src/lib.rs

@@ -145,6 +145,12 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
     };
     let emit_debug_info = matches.is_present(FLAG_DEBUG);
 
+    let link_type = if matches.is_present(FLAG_LIB) {
+        LinkType::Dylib
+    } else {
+        LinkType::Executable
+    };
+
     let path = Path::new(filename).canonicalize().unwrap();
     let src_dir = path.parent().unwrap().canonicalize().unwrap();
 
@@ -174,7 +180,7 @@ pub fn build(target: &Triple, matches: &ArgMatches, config: BuildConfig) -> io::
         path,
         opt_level,
         emit_debug_info,
-        LinkType::Executable,
+        link_type,
     );
 
     match res_binary_path {

compiler/build/src/link.rs

@@ -12,8 +12,10 @@ use target_lexicon::{Architecture, OperatingSystem, Triple};
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum LinkType {
-    Executable,
-    Dylib,
+    // These numbers correspond to the --lib flag; if it's present
+    // (e.g. is_present returns `1 as bool`), this will be 1 as well.
+    Executable = 0,
+    Dylib = 1,
 }
 
 /// input_paths can include the host as well as the app. e.g. &["host.o", "roc_app.o"]