Skip Python installation paths for build-tree Python

docs/releasenotes/13.1.0.rst

@@ -54,3 +54,11 @@ Blurring I;16 images
 
 :py:class:`~PIL.ImageFilter.GaussianBlur` and :py:class:`~PIL.ImageFilter.BoxBlur`
 now support I;16, I;16L, I;16B and I;16N images.
+
+Respect Python build-tree configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When building against a Python build tree, Pillow now skips the extra library and
+include directory lookups based on Python's installation prefix. This avoids
+adding unprefixed installation paths in cross-compilation environments. Builds
+against an installed Python retain the existing lookups.

setup.py

@@ -572,16 +572,20 @@ def build_extensions(self) -> None:
                 for d in os.environ[k].split(os.path.pathsep):
                     _add_directory(library_dirs, d)
 
-        _add_directory(
-            library_dirs,
-            (sys.prefix == sys.base_prefix and sysconfig.get_config_var("LIBDIR"))
-            or os.path.join(sys.prefix, "lib"),
-        )
-        _add_directory(
-            include_dirs,
-            (sys.prefix == sys.base_prefix and sysconfig.get_config_var("INCLUDEDIR"))
-            or os.path.join(sys.prefix, "include"),
-        )
+        if not sysconfig.is_python_build():
+            _add_directory(
+                library_dirs,
+                (sys.prefix == sys.base_prefix and sysconfig.get_config_var("LIBDIR"))
+                or os.path.join(sys.prefix, "lib"),
+            )
+            _add_directory(
+                include_dirs,
+                (
+                    sys.prefix == sys.base_prefix
+                    and sysconfig.get_config_var("INCLUDEDIR")
+                )
+                or os.path.join(sys.prefix, "include"),
+            )
 
         #
         # add platform directories