chore(build/common.prf): prefer ../generated_cpp & keep Qt5 fallbacks

Default to `../generated_cpp` (generator output). If missing, probe the
exact-version dir (`generated_cpp_<MAJOR><MINOR>`, e.g., 515) and then the
legacy Qt5 fallbacks.

Also allow override via:

```
qmake PYTHONQT_GENERATED_PATH=/abs/path
```

When wrappers are absent, emit a concise error that lists the searched
paths and indicates how to generate them.

Author: jcfr

build/common.prf

@@ -17,42 +17,58 @@ CONFIG(debug, debug|release) {
   TARGET = $${TARGET}_d 
 }
 
-# Try files that are generated by the user:
-PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp
+# Allow override (e.g., qmake PYTHONQT_GENERATED_PATH=/abs/path)
+isEmpty(PYTHONQT_GENERATED_PATH) {
+  PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp
+}
 
+# Fallbacks to checked-in wrappers
 !exists($$PYTHONQT_GENERATED_PATH) {
-  # If no files are generated, try the checked-in wrappers:
-  PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_$${QT_MAJOR_VERSION}$${QT_MINOR_VERSION}
+  # Exact version directory (e.g., generated_cpp_515 or generated_cpp_69)
+  PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_$${QT_MAJOR_VERSION}$${QT_MINOR_VERSION}
+  exists($$PYTHONQT_GENERATED_PATH_VERSIONED) {
+    PYTHONQT_GENERATED_PATH = $$PYTHONQT_GENERATED_PATH_VERSIONED
+  }
+}
 
-  !exists($$PYTHONQT_GENERATED_PATH) {
-    # For Qt5 we know that the older generated wrappers work with the later
-    # versions, even (apparently) Qt5.15, so:
-    equals(QT_MAJOR_VERSION, 5) {
-      # Qt5: have 5.0, 5.3, 5.4, 5.6 and 5.11 at present:
-      lessThan(QT_MINOR_VERSION, 3) { # 5.1, 5.2
-        PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_50
-      }
-      else: lessThan(QT_MINOR_VERSION, 6) { # 5.5
-        PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_54
-      }
-      else: lessThan(QT_MINOR_VERSION, 11) { # 5.7, 5.8, 5.9, 5.10
-        PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_56
-      }
-      else: lessThan(QT_MINOR_VERSION, 15) { # 5.11, 5.12, 5.13, 5.14
-        PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_511
-      }
-      else { # 5.15
-        # LATEST Qt5 generated files:
-        PYTHONQT_GENERATED_PATH = $$PWD/../generated_cpp_515
-      }
+!exists($$PYTHONQT_GENERATED_PATH) {
+  # For Qt5 we know that the older generated wrappers work with the later
+  # versions, even (apparently) Qt5.15, so:
+  equals(QT_MAJOR_VERSION, 5) {
+    # Qt5: have 5.0, 5.3, 5.4, 5.6 and 5.11 at present:
+    lessThan(QT_MINOR_VERSION, 3) { # 5.1, 5.2
+      PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_50
     }
-
-    !exists($$PYTHONQT_GENERATED_PATH) {
-      error("No generated sources exist for Qt$${QT_VERSION}")
+    else: lessThan(QT_MINOR_VERSION, 6) { # 5.3, 5.4, 5.5
+      PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_54
+    }
+    else: lessThan(QT_MINOR_VERSION, 11) { # 5.6, 5.7, 5.8, 5.9, 5.10
+      PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_56
+    }
+    else: lessThan(QT_MINOR_VERSION, 15) { # 5.11, 5.12, 5.13, 5.14
+      PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_511
+    }
+    else { # >= 5.15
+      PYTHONQT_GENERATED_PATH_VERSIONED = $$PWD/../generated_cpp_515
+    }
+    exists($$PYTHONQT_GENERATED_PATH_VERSIONED) {
+      PYTHONQT_GENERATED_PATH = $$PYTHONQT_GENERATED_PATH_VERSIONED
     }
   }
 }
 
+!exists($$PYTHONQT_GENERATED_PATH) {
+  error("No generated wrapper sources for Qt $${QT_VERSION}.\n"
+        + "Looked in:\n"
+        + "  - $$PWD/../generated_cpp\n"
+        + "  - $$PWD/../generated_cpp_$${QT_MAJOR_VERSION}$${QT_MINOR_VERSION} (e.g., generated_cpp_515)\n"
+        + "Generate them first:\n"
+        + "  qmake CONFIG+=generator_only && make\n"
+        + "  cd generator && ./pythonqt_generator\n"
+        + "Or pass an explicit path:\n"
+        + "  qmake PYTHONQT_GENERATED_PATH=/abs/path")
+}
+
 !build_pass {
   message("Qt version: Qt$${QT_VERSION}")
   message("Using generated sources files from $${PYTHONQT_GENERATED_PATH}")