Rework libidn2 detection

Clean up the parts of configure.in responsible for handling libidn2
detection and adjust other pieces of the build system to match these
cleanups:

  - use pkg-config when --with-libidn2 is used without an explicit path,

  - look for idn2_to_ascii_lz() rather than idn2_to_ascii_8z() as the
    former is used in BIND while the latter is not,

  - do not look for idn2_to_unicode_8zlz() as it is present in all
    libidn2 versions which have idn2_to_ascii_lz(),

  - check whether the <idn2.h> header is usable,

  - set LDFLAGS in the Makefile for dig so that, if specified, the
    requested libidn2 path is used when linking with libidn2,

  - override CPPFLAGS when looking for libidn2 components so that the
    configure script does not produce warnings when libidn2 is not
    installed system-wide,

  - merge the AS_CASE() call into the AS_IF() call below it to simplify
    code,

  - indicate the default value of --with-libidn2 in "./configure --help"
    output,

  - use $with_libidn2 rather than $use_libidn2 to better match the name
    of the configure script argument,

  - stop differentiating between IDN "in" and "out" support, i.e. make
    dig either support libidn2 or not; remove WITH_* Autoconf macros and
    use a new one, HAVE_LIBIDN2, to determine whether libidn2 support
    should be enabled.

config.h.in

@@ -293,6 +293,9 @@ int sigwait(const unsigned int *set, int *sig);
 /* Define to 1 if you have the <gssapi_krb5.h> header file. */
 #undef HAVE_GSSAPI_KRB5_H
 
+/* Define to 1 if you have the <idn2.h> header file. */
+#undef HAVE_IDN2_H
+
 /* Define to 1 if you have the if_nametoindex function. */
 #undef HAVE_IF_NAMETOINDEX
 
@@ -323,6 +326,9 @@ int sigwait(const unsigned int *set, int *sig);
 /* Define to 1 if you have the `c_r' library (-lc_r). */
 #undef HAVE_LIBC_R
 
+/* Define if libidn2 was found */
+#undef HAVE_LIBIDN2
+
 /* Define to 1 if you have the `nsl' library (-lnsl). */
 #undef HAVE_LIBNSL
 
@@ -620,15 +626,6 @@ int sigwait(const unsigned int *set, int *sig);
 /* Define to enable very verbose query trace logging. */
 #undef WANT_QUERYTRACE
 
-/* define if IDN output support is to be included. */
-#undef WITH_IDN_OUT_SUPPORT
-
-/* define if IDN input support is to be included. */
-#undef WITH_IDN_SUPPORT
-
-/* define if libidn2 support is to be included. */
-#undef WITH_LIBIDN2
-
 /* Define WORDS_BIGENDIAN to 1 if your processor stores words with the most
    significant byte first (like Motorola and SPARC, unlike Intel). */
 #if defined AC_APPLE_UNIVERSAL_BUILD