Use Win32 MessageBox for asserts on cygwin

src/assert.cc

@@ -3,6 +3,10 @@
 #include "exception.hh"
 #include "debug.hh"
 
+#if defined(__CYGWIN__)
+#include <windows.h>
+#endif
+
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -24,8 +28,19 @@ void on_assert_failed(const char* message)
     String debug_info = "pid: " + to_string(getpid());
     write_debug("assert failed: '"_str + message + "' " + debug_info);
 
-    int res = system(("xmessage -buttons 'quit:0,ignore:1' '"_str +
-                      message + "\n[Debug Infos]\n" + debug_info + "'").c_str());
+    const auto msg = message + "\n[Debug Infos]\n"_str + debug_info;
+#if defined(__CYGWIN__)
+    int res = MessageBox(NULL, msg.c_str(), "Kakoune: assert failed",
+                         MB_OKCANCEL | MB_ICONERROR);
+    switch (res)
+    {
+    case IDCANCEL:
+        throw assert_failed(message);
+    case IDOK:
+        return;
+    }
+#else
+    int res = system(("xmessage -buttons 'quit:0,ignore:1' '" + msg + "'").c_str());
     switch (res)
     {
     case -1:
@@ -34,6 +49,7 @@ void on_assert_failed(const char* message)
     case 1:
         return;
     }
+#endif
 }
 
 }