Notmuch dynamic library can now be loaded on macos

On macos, the name of dynamic libraries is a little bit different than on Unix.
The code was looking for a `libnotmuch.so.5` library, while on macos
it's installed as `libnotmuch.5.dylib`.

This commit uses cfg attribute to conditionally change the library name.

Closes #106

melib/src/backends.rs

@@ -156,7 +156,11 @@ impl Backends {
         }
         #[cfg(feature = "notmuch_backend")]
         {
-            if libloading::Library::new("libnotmuch.so.5").is_ok() {
+            #[cfg(target_os = "unix")]
+            let dlpath = "libnotmuch.so.5";
+            #[cfg(target_os = "macos")]
+            let dlpath = "libnotmuch.5.dylib";
+            if libloading::Library::new(dlpath).is_ok() {
                 b.register(
                     "notmuch".to_string(),
                     Backend {

melib/src/backends/notmuch.rs

@@ -307,7 +307,11 @@ impl NotmuchDb {
         _is_subscribed: Box<dyn Fn(&str) -> bool>,
         event_consumer: BackendEventConsumer,
     ) -> Result<Box<dyn MailBackend>> {
-        let lib = Arc::new(libloading::Library::new("libnotmuch.so.5")?);
+        #[cfg(target_os = "unix")]
+        let dlpath = "libnotmuch.so.5";
+        #[cfg(target_os = "macos")]
+        let dlpath = "libnotmuch.5.dylib";
+        let lib = Arc::new(libloading::Library::new(dlpath)?);
         let path = Path::new(s.root_mailbox.as_str()).expand();
         if !path.exists() {
             return Err(MeliError::new(format!(