python: update PYTHON_FOR_BUILD (based on python3)

PYTHON_FOR_BUILD is used to build the target python.
Nomally Python scripts detect fine the Python host interpreter
to use for building the Python target interpreter.
But, let's not leave it up to chance anymore.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python/Makefile

@@ -122,6 +122,13 @@ ifeq ($(CONFIG_IPV6),y)
 	ENABLE_IPV6 += --enable-ipv6
 endif
 
+PYTHON_FOR_BUILD:= \
+	_PYTHON_PROJECT_BASE=$(PKG_BUILD_DIR) \
+	_PYTHON_HOST_PLATFORM=linux2 \
+	PYTHONPATH="$(PKG_BUILD_DIR)/Lib:$(PKG_BUILD_DIR)/build/lib.linux2-$(PYTHON_VERSION)" \
+	_PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata \
+	$(HOST_PYTHON_BIN)
+
 CONFIGURE_ARGS+= \
 	--sysconfdir=/etc \
 	--enable-shared \
@@ -129,6 +136,7 @@ CONFIGURE_ARGS+= \
 	--with-threads \
 	--with-system-ffi \
 	--without-pymalloc \
+	PYTHON_FOR_BUILD="$(PYTHON_FOR_BUILD)" \
 	$(ENABLE_IPV6) \
 	CONFIG_SITE="$(PKG_BUILD_DIR)/config.site" \
 	OPT="$(TARGET_CFLAGS)"