[xmlsec] docs directory install

[Aleksey Sanin]

I was not able to find a way to make "--docdir" option because autoconf 
has built in
support only for "--with-*" and "--enable-*" options and all the 
"--prefix", "--libdir",
etc. are hard coded. Instead, I have added "XMLSEC_DOCDIR" environment
variable support. You can use it in ./configure command line thus it'll 
be written
to config.cache like this:
            ./configure --prefix=/usr XMLSEC_DOCDIR=/usr/share/xmlsec1/doc
By default, the previous path "$prefix/share/doc/xmlsec1" is used.

Next I have removed all the *.sgml files from installation. Now only 
*.html files
and images are installed.

These changes are checked in and will be included in next release which
should happen tomorrow.

Aleksey