[tlbuild] Old versions of autoconf

[Ken Brown]

Most of the texlive build system uses autoconf 2.68, but there are still 
a few places where older versions were used.  Two in particular affect 
the Cygwin build, because the autoconf mmap test (AC_FUNC_MMAP) was 
broken until autoconf 2.65 and incorrectly failed on Cygwin.  Here are 
the relevant excerpts of build.log:

/tmp/tl2011/source/Work/libs/freetype/../../../libs/freetype/freetype-1.5/configure
[...]
checking for working mmap... no
[...]
../../../../utils/dialog/dialog-1.1-20110302/configure
[...]
checking for working mmap... no
[...]
../../../texk/dvipng/configure
[...]
checking for working mmap... (cached) no
[...]

Note that freetype-1.5/configure was generated by autoconf 2.61a, and 
dialog-1.1-20110302/configure was generated by autoconf 2.52.20101002. 
There's no problem with texk/dvipng/configure in itself, but it uses the 
cached incorrect result.

I can easily work around this by exporting

   ac_cv_func_mmap_fixed_mapped=yes

before doing the build, but it seems better to bring the build system up 
to date if possible.

Thanks.

Ken