python3: use _python_sysroot for cross-compilation

This patch, taken from buildroot, avoids the use of host paths when
compiling third-party extensions.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

lang/python/python3/Makefile

@@ -14,7 +14,7 @@ PYTHON_VERSION:=$(PYTHON3_VERSION)
 PYTHON_VERSION_MICRO:=$(PYTHON3_VERSION_MICRO)
 
 PKG_NAME:=python3
-PKG_RELEASE:=13
+PKG_RELEASE:=14
 PKG_VERSION:=$(PYTHON_VERSION).$(PYTHON_VERSION_MICRO)
 
 PKG_SOURCE:=Python-$(PKG_VERSION).tar.xz

lang/python/python3/patches/008-distutils-use-python-sysroot.patch

@@ -0,0 +1,67 @@
+From e359a7a3c4f9e70360a068bef19c95938fdacede Mon Sep 17 00:00:00 2001
+From: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+Date: Wed, 23 Dec 2015 11:33:14 +0100
+Subject: [PATCH] Adjust library/header paths for cross-compilation
+
+When cross-compiling third-party extensions, the get_python_inc() or
+get_python_lib() can be called, to return the path to headers or
+libraries. However, they use the sys.prefix of the host Python, which
+returns incorrect paths when cross-compiling (paths pointing to host
+headers and libraries).
+
+In order to fix this, we introduce the _python_sysroot, _python_prefix
+and _python_exec_prefix variables, that allow to override these
+values, and get correct header/library paths when cross-compiling
+third-party Python modules.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+---
+ Lib/distutils/command/build_ext.py |  5 ++++-
+ Lib/distutils/sysconfig.py         | 15 +++++++++++----
+ 2 files changed, 15 insertions(+), 5 deletions(-)
+
+diff --git a/Lib/distutils/command/build_ext.py b/Lib/distutils/command/build_ext.py
+index 74de782d8a..d0c847b365 100644
+--- a/Lib/distutils/command/build_ext.py
++++ b/Lib/distutils/command/build_ext.py
+@@ -234,7 +234,10 @@ class build_ext(Command):
+         if (sysconfig.get_config_var('Py_ENABLE_SHARED')):
+             if not sysconfig.python_build:
+                 # building third party extensions
+-                self.library_dirs.append(sysconfig.get_config_var('LIBDIR'))
++                libdir = sysconfig.get_config_var('LIBDIR')
++                if "_python_sysroot" in os.environ:
++                    libdir = os.environ.get("_python_sysroot") + libdir
++                self.library_dirs.append(libdir)
+             else:
+                 # building python standard extensions
+                 self.library_dirs.append('.')
+diff --git a/Lib/distutils/sysconfig.py b/Lib/distutils/sysconfig.py
+index 2bcd1dd288..422c13fa4f 100644
+--- a/Lib/distutils/sysconfig.py
++++ b/Lib/distutils/sysconfig.py
+@@ -17,10 +17,17 @@ import sys
+ from .errors import DistutilsPlatformError
+ 
+ # These are needed in a couple of spots, so just compute them once.
+-PREFIX = os.path.normpath(sys.prefix)
+-EXEC_PREFIX = os.path.normpath(sys.exec_prefix)
+-BASE_PREFIX = os.path.normpath(sys.base_prefix)
+-BASE_EXEC_PREFIX = os.path.normpath(sys.base_exec_prefix)
++if "_python_sysroot" in os.environ:
++    _sysroot=os.environ.get('_python_sysroot')
++    PREFIX = os.path.normpath(_sysroot + os.environ.get('_python_prefix'))
++    EXEC_PREFIX = os.path.normpath(_sysroot + os.environ.get('_python_exec_prefix'))
++    BASE_PREFIX = PREFIX
++    BASE_EXEC_PREFIX = EXEC_PREFIX
++else:
++    PREFIX = os.path.normpath(sys.prefix)
++    EXEC_PREFIX = os.path.normpath(sys.exec_prefix)
++    BASE_PREFIX = os.path.normpath(sys.base_prefix)
++    BASE_EXEC_PREFIX = os.path.normpath(sys.base_exec_prefix)
+ 
+ # Path to the base directory of the project. On Windows the binary may
+ # live in project/PCbuild/win32 or project/PCbuild/amd64.
+-- 
+2.13.5
+