configure: Add check for the doc tools

configure: Add check for the doc tools

Added checking existence of the doc files converters.
If the XXX tool exists then HAVE_XXX:=y will be written
to the Config file. Example of the configure script output:

TC schedulers
 ATM	no
 IPT	using xtables
 IPSET  yes

iptables modules directory: /usr/lib/iptables
libc has setns: yes
SELinux support: no

Docs
 latex: no
 WARNING: no docs can be built from LaTeX files
 sgml2html: yes

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>