Handle only llvm-objdump being available

llvm-objdump currently doesn't appear to output the Windows subsystem, so
that part of the test needs to be skipped.

test cases/windows/16 gui app/meson.build

@@ -17,6 +17,10 @@ console_prog = executable('console_prog', 'console_prog.c', gui_app: false)
 
 tester = find_program('gui_app_tester.py')
 
-tool = find_program('objdump', 'dumpbin')
-test('is_gui', tester, args: [tool.path(), gui_prog, '2'])
-test('not_gui', tester, args: [tool.path(), console_prog, '3'])
+tool = find_program('objdump', 'dumpbin', required: false)
+# TODO: when 'llvm-objdump -f' emits the subsystem type, we could use that also
+
+if tool.found()
+  test('is_gui', tester, args: [tool.path(), gui_prog, '2'])
+  test('not_gui', tester, args: [tool.path(), console_prog, '3'])
+endif