Windows port: Use an improved python.m4

Windows port: Use an improved python.m4

- AM_CHECK_PYTHON_HEADERS macro now allows PYTHON_INCLUDES to be overridden
  from an environment variable and
- the new AM_CHECK_PYTHON_LIBS macro to check for ability to link against
  libpython. This also allows PYTHON_LIBS and PYTHON_LIB_LOC to be overridden
  from their respective environment variables.

This allows gobject-introspection to be built with MinGW/MSYS by doing:

PYTHON_DIR="/c/Python27"
SRC_DIR="/d/dev/gnome.org/checkout/gobject-introspection/src"
...
PYTHON_INCLUDES="-I${PYTHON_DIR}/include/" \
PYTHON_LIBS="-L${PYTHON_DIR}/libs/ -lpython${PYTHON_VERSION}" \
PYTHON_LIB_LOC="${PYTHON_DIR}/libs/" \
"${SRC_DIR}/configure" \

https://bugzilla.gnome.org/show_bug.cgi?id=620566