filter/texttotext.c: link with libiconv if needed

filter/texttotext.c: link with libiconv if needed

texttotext.c uses iconv so it should link with libiconv on platforms
where it is a separate library (e.g. uClibc-ng without built-in NLS)
otherwise texttotext fails to link:

  CCLD     texttotext
[...]/ld: texttotext-texttotext.o: in function `main':
texttotext.c:(.text.startup+0xde0): undefined reference to `libiconv_open'
[...]/ld: texttotext.c:(.text.startup+0xf9d): undefined reference to `libiconv'
[...]/ld: texttotext.c:(.text.startup+0xfd6): undefined reference to `libiconv'
[...]/ld: texttotext.c:(.text.startup+0x16c3): undefined reference to `libiconv_close'

Modify autogen.sh to call autopoint, which adds the libiconv discovery.
It also creates a "po" skeleton but we can discard it, since it is not
really necessary.

Fixes: https://bugs.busybox.net/show_bug.cgi?id=12031
Signed-off-by: Carlos Santos <unixmania@gmail.com>