package/pkg-python: set _PYTHON_PROJECT_BASE

This variable is supposed to be set when cross compiling and is
now required by setuptools to be set correctly after the latest
version bump.

Not much reference for that variable, exept for:
    https://github.com/python/cpython/blob/v3.10.2/Lib/sysconfig.py#L157-L159

Fixes:
http://autobuild.buildroot.net/results/2e8/2e8093e68d04e8cbb20b095068b24c8bbe5f69ad
http://autobuild.buildroot.net/results/9e3/9e3cfddb651a59d7f2d04bc88e6fff2a06e9b465

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-python.mk

@@ -33,6 +33,7 @@ PKG_PYTHON_DISTUTILS_ENV = \
 	LDSHARED="$(TARGET_CROSS)gcc -shared" \
 	PYTHONPATH="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_PATH),$(PYTHON_PATH))" \
 	PYTHONNOUSERSITE=1 \
+	_PYTHON_PROJECT_BASE="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_DIR),$(PYTHON_DIR))" \
 	_PYTHON_SYSCONFIGDATA_NAME="$(PKG_PYTHON_SYSCONFIGDATA_NAME)" \
 	_python_sysroot=$(STAGING_DIR) \
 	_python_prefix=/usr \
@@ -60,6 +61,7 @@ HOST_PKG_PYTHON_DISTUTILS_INSTALL_OPTS = \
 
 # Target setuptools-based packages
 PKG_PYTHON_SETUPTOOLS_ENV = \
+	_PYTHON_PROJECT_BASE="$(if $(BR2_PACKAGE_PYTHON3),$(PYTHON3_DIR),$(PYTHON_DIR))" \
 	_PYTHON_SYSCONFIGDATA_NAME="$(PKG_PYTHON_SYSCONFIGDATA_NAME)" \
 	PATH=$(BR_PATH) \
 	$(TARGET_CONFIGURE_OPTS) \