libQGLViewer: add new package (#20164)

lib/spack/docs/build_systems/qmakepackage.rst

@@ -108,6 +108,19 @@ override the ``qmake_args`` method like so:
 
 This method can be used to pass flags as well as variables.
 
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``*.pro`` file in a sub-directory
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If the ``*.pro`` file used to tell QMake how to build the package is
+found in a sub-directory, you can tell Spack to run all phases in this
+sub-directory by adding the following to the package:
+
+.. code-block:: python
+
+   build_directory = 'src'
+
+
 ^^^^^^^^^^^^^^^^^^^^^^
 External documentation
 ^^^^^^^^^^^^^^^^^^^^^^