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.

The _python_sysroot variable is also used to prefix the LIBDIR value
taken from the sysconfigdata module.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Lib/distutils/sysconfig.py
===================================================================
--- a/Lib/distutils/sysconfig.py
+++ b/Lib/distutils/sysconfig.py
@@ -19,8 +19,13 @@
 from distutils.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)
+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'))
+else:
+    PREFIX = os.path.normpath(sys.prefix)
+    EXEC_PREFIX = os.path.normpath(sys.exec_prefix)
 
 # Path to the base directory of the project. On Windows the binary may
 # live in project/PCBuild9.  If we're dealing with an x64 Windows build,
Index: b/Lib/distutils/command/build_ext.py
===================================================================
--- a/Lib/distutils/command/build_ext.py
+++ b/Lib/distutils/command/build_ext.py
@@ -237,7 +237,10 @@
         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('.')