slang: clean up options and dependencies

slang: clean up options and dependencies

The AC_PATH_XTRA check for X11 headers is never actually used, so patch that out
and remove the options in EXTRA_OECONF.

Move pcre/png/zlib toggles to PACKAGECONFIG, retaining the behaviour that only
PCRE is enabled by default.

Add missing libiconv dependency.

(From OE-Core rev: 3587053646c34002fa18b87834516ce27fbd0788)

(From OE-Core rev: 200cc00b048dbef11b01c6402d6525559de76306)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>