Fix bug in `ExeComponent.mk_uninstall()` in the build system

which would try to uninstall components that were never installed.

This bug would cause the following line to be emitted in the
``Makefile`` under the ``uninstall`` rule even though there was
no corresponding rule to install the file under the ``install`` rule.

```
@rm -f $(DESTDIR)$(PREFIX)/bin/test-z3$(EXE_EXT)
```

scripts/mk_util.py

@@ -1094,6 +1094,7 @@ class ExeComponent(Component):
             MakeRuleCmd.install_files(out, exefile, os.path.join('bin', exefile))
 
     def mk_uninstall(self, out):
+        if self.install:
             exefile = '%s$(EXE_EXT)' % self.exe_name
             MakeRuleCmd.remove_installed_files(out, os.path.join('bin', exefile))