chore(qmake): add toggles to build only the generator or exclude it

Introduce two qmake CONFIG flags to control subdir builds:

- CONFIG+=generator_only       -> build just the wrapper generator
- CONFIG+=exclude_generator    -> build everything except the generator

Default remains unchanged (all subdirs). This enables CI to generate
wrappers first, then build without the generator to avoid redundant work.

Author: jcfr

PythonQt.pro

@@ -1,6 +1,24 @@
 TEMPLATE = subdirs
 
-SUBDIRS = generator src extensions tests examples
+# Define the full set of subdirectories, including the generator.
+# This list is used as the default build target.
+ALL_SUBDIRS = generator src extensions tests examples
+
 tests.depends += src extensions
 extensions.depends += src
 examples.depends += src extensions
+
+# The 'generator_only' config can be used to override the default.
+CONFIG(generator_only) {
+    # If the generator_only config is set, override the SUBDIRS list
+    # to only include the generator.
+    SUBDIRS = generator
+} else: CONFIG(exclude_generator) {
+    # If the 'generator_only' option is not present, but 'exclude_generator' is,
+    # then remove the generator from the default list.
+    SUBDIRS = $$ALL_SUBDIRS
+    SUBDIRS -= generator
+} else {
+    # If neither special option is set, use the default build which includes everything.
+    SUBDIRS = $$ALL_SUBDIRS
+}