Updating m4 files

[Rik]

8/10/12

John,

I've been overhauling the m4 files to try and make Octave's build system
cleaner.  I've removed deprecated macros from acinclude.m4.  But, we also
ship some other m4 files such as ax_pthread.m4, ax_blas.m4, etc.  What do
you think about updating to the latest versions of these m4 files?  I've
only taken a look at ax_pthread.m4 but it seems to have gotten rid of most
of the deprecated macros and is probably less buggy.

--Rik