add desktop file and install it with unison-gui

Taken from the Flatpak repo
https://github.com/flathub/io.github.bcpierce00.unison
and originally based on the Fedora rpm source.

Author: mistmist

data/io.github.bcpierce00.unison.desktop

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: MIT-0
+[Desktop Entry]
+Type=Application
+Exec=unison-gui
+Name=Unison
+GenericName=File Synchronizer
+Comment=Multi-master File synchronization tool
+Terminal=false
+Icon=io.github.bcpierce00.unison
+StartupNotify=true
+Categories=Utility;

src/make_tools.ml

@@ -576,6 +576,7 @@ let install () =
   let mandir = "MANDIR" <--? datarootdir ^ "/man" in
   let man1dir = "MAN1DIR" <--? mandir ^ "/man1" in
   let manext = "MANEXT" <--? ".1" in
+  let desktopdir = "DESKTOPDIR" <--? datarootdir ^ "/applications" in
 
   let install_if_exists dir name dest =
     if exists dir name then exec
@@ -592,6 +593,10 @@ let install () =
     exec [install; "-d"; destdir ^ man1dir];
     exec [install_data; "man/unison.1"; destdir ^ man1dir ^ "/unison" ^ manext]
   end;
+  if exists "src" "unison-gui" then begin
+    exec [install; "-d"; destdir ^ desktopdir];
+    exec [install_data; "data/io.github.bcpierce00.unison.desktop"; destdir ^ desktopdir ^ "/io.github.bcpierce00.unison.desktop"]
+  end;
   if exists "src/uimac/build/Default" "Unison.app" then begin
     print_endline ("!!! The GUI for macOS has been built but will NOT be \
       installed automatically. You can find the built GUI package at " ^