build: introduce SAMBA_CHECK_PYTHON_HEADERS

build: introduce SAMBA_CHECK_PYTHON_HEADERS

This function is a wrapper around waf's check_python_header.
It avoids searching more than once for the headers bringing a small
speed improvement and a better lisibility of the logs.

But it's mainly to avoid a nasty bug when python libraries are in path
pointed by python_LIBPL (ie. /usr/local/lib/python2.6/config/) instead
of python_LIBDIR (ie. /usr/local/lib).

On the first call waf will correctly find that in order to link with
python libs it needs to add -L$python_LIBPL.

But on the next calls of check_python_headers, waf will use both the
current library path value (ie. -L/usr/local/lib/python2.6/config) and
-L$python_LIBDIR (ie. /usr/local/lib/) which will make him beleive that
python libraries are in $python_LIBDIR which at the end will make the
final link test fails in check_python_headers as it will not use the
good directory.

So by avoiding calling check_python_headers more than once we avoid
making waf fooling itself.