Fix issue:
Added additional check for this place.
Reason: If slang installed into /usr/local path, then ./configure
script stopped with 'Slang library not found' error
Also added new two options to configure script:
--with-slang-includes=PATH
--with-slang-libs=PATH
Now libslang finds in (by order):
* --with-slang-includes and --with-slang-libs
* /usr
* /usr/local
Signed-off-by: Slava Zanko <slavazanko@gmail.com>
