Handling of --without-* in gtkpod's configure script

Hello,

the check for --without-* is wrong for curl and for OGG and when
running ./configure without arguments I get this error:
configure: error: Ogg/Vorbis support explicitly requested but
ogg/vorbis libs could not be found

I have installed only the prerequisites for curl, not for OGG nor FLAC
(but the check for FLAC is different):

AC_ARG_WITH(curl, AS_HELP_STRING([--without-curl],[Disable coverart
download support]))
if test "x$with_curl" != "xno" -a "x$have_curl" = "xno"; then
	AC_MSG_ERROR([curl support explicitly requested but curl could not be found])
[...]
AC_ARG_WITH(ogg, AS_HELP_STRING([--without-ogg],[Disable Ogg/Vorbis support]))
if test "x$with_ogg" != "xno" -a "x$have_vorbis" = "xno"; then
    AC_MSG_ERROR([Ogg/Vorbis support explicitly requested but
ogg/vorbis libs could not be found])
[...]
AC_ARG_WITH(flac, AS_HELP_STRING([--without-flac],[Disable FLAC support]))
if test "xwith_flac" = "xyes" -a "x$have_flac" = "xno"; then
	AC_MSG_ERROR([FLAC support explicitly requested but flac libs could
not be found])
[...]

It's not stated in the docs
http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/External-Software.html
but it seems that the variable isn't set when the argument is not
given on ./configure's command line (the docs say that it's "yes" or
"no" when it is given) and all the examples in that page set a default
value in the [action-if-not-given] and l ibgpod does the same

since there are several ways to fix that, it's better if phantomjinx
can look at it
-- 
Daniele Forsi

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