Fix get_python_inc() for cross-compilation

When we are cross compiling, doing os.path.dirname(sys.executable) to
get the build directory is incorrect, because we're executing the host
Python to build things for the target. Instead, we should use the
project_base variable.

This fixes cross-compilation, which was adding incorrect header paths
pointing to the location where the host Python was built:

/home/thomas/projets/buildroot/output/host/usr/bin/arm-none-linux-gnueabi-gcc -fPIC -fno-strict-aliasing \
  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -pipe -Os -DNDEBUG -g -O3 -Wall -Wstrict-prototypes \
  -I/usr/include -I. -IInclude -I./Include -I/home/thomas/projets/buildroot/output/host/usr/arm-buildroot-linux-gnueabi/sysroot/usr/include \
  -I/home/thomas/projets/buildroot/output/host/usr/bin/Include -I/home/thomas/projets/buildroot/output/host/usr/bin \
  -c /home/thomas/projets/buildroot/output/build/python-2.7.6/Modules/_struct.c \
  -o build/temp.linux2-arm-2.7/home/thomas/projets/buildroot/output/build/python-2.7.6/Modules/_struct.o

This patch allows to fix the
/home/thomas/projets/buildroot/output/host/usr/bin/Include and
/home/thomas/projets/buildroot/output/host/usr/bin paths that are
incorrectly added to the header paths.

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
@@ -79,7 +79,7 @@
 
     if os.name == "posix":
         if python_build:
-            buildir = os.path.dirname(sys.executable)
+            buildir = project_base
             if plat_specific:
                 # python.h is located in the buildir
                 inc_dir = buildir