New .spec file to build RPM

[Carlo Contavalli]

I've finally added a .spec file to build RPMs, kindly provided by Minfrin,
see: https://github.com/ccontavalli/mod-xslt/issues/3

I've also updated the wiki about how to install on Centos, if anyone
could try it out, that'd be great.
I've verified on a centos VM, and seems to be working well.

For instructions, see:
https://github.com/ccontavalli/mod-xslt/wiki/Installing-on-CentOS

The section:
"using rpmbuild"

Note that you have to download mod-xslt 1.4.2, and note that the
addition of the .spec file and well, updating autoconf, is the only
change in that version.

Thanks,
Carlo