setup.py: do not add invalid header locations

This piece of code incorrectly adds /usr/include to
self.compiler.include_dirs, and results in the following invalid
compilation line:

/home/thomas/projets/buildroot/output/host/usr/bin/arm-none-linux-gnueabi-gcc
  -fPIC -Wno-unused-result -Werror=declaration-after-statement -DNDEBUG -g
  -O3 -Wall -Wstrict-prototypes -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
  -D_FILE_OFFSET_BITS=64 -pipe -Os
  -I./Include -I/usr/include -I. -IInclude
  -I/home/thomas/projets/buildroot/output/host/usr/arm-buildroot-linux-gnueabi/sysroot/usr/include
  -I/home/thomas/projets/buildroot/output/build/python3-3.4.0b1/Include
  -I/home/thomas/projets/buildroot/output/build/python3-3.4.0b1
  -c /home/thomas/projets/buildroot/output/build/python3-3.4.0b1/Modules/_struct.c
  -o build/temp.linux-arm-3.4/home/thomas/projets/buildroot/output/build/python3-3.4.0b1/Modules/_struct.o
cc1: warning: include location "/usr/include" is unsafe for cross-compilation [-Wpoison-system-directories]

The -I/usr/include is wrong when cross compiling, so we disable adding
INCLUDEDIR and LIBDIR from the host when cross compiling.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Index: b/setup.py
===================================================================
--- a/setup.py
+++ b/setup.py
@@ -493,7 +493,8 @@
                         add_dir_to_list(dir_list, directory)
 
         if os.path.normpath(sys.base_prefix) != '/usr' \
-                and not sysconfig.get_config_var('PYTHONFRAMEWORK'):
+                and not sysconfig.get_config_var('PYTHONFRAMEWORK') \
+                and not cross_compiling:
             # OSX note: Don't add LIBDIR and INCLUDEDIR to building a framework
             # (PYTHONFRAMEWORK is set) to avoid # linking problems when
             # building a framework with different architectures than