Autodetect libidn/libidn2 support with pkg-config

libidn2 >= 2.0.3 must be used, or else it would silently remove
characters not valid in hostnames like "/" and "_".
For details see #50 .

For more information about IDNA2003 vs. IDNA2008 and TR46 see:
http://nmav.gnutls.org/2017/04/the-mess-with-internationalized-domain.html
http://unicode.org/faq/idn.html
https://fedoraproject.org/wiki/Changes/IDNA2008

Makefile

@@ -32,14 +32,12 @@ ifdef LOCALEDIR
 DEFS += -DLOCALEDIR=\"$(BASEDIR)$(prefix)/share/locale\"
 endif
 
-ifdef HAVE_LIBIDN2
-whois_LDADD += -lidn2
-DEFS += -DHAVE_LIBIDN2
-else
-ifdef HAVE_LIBIDN
-whois_LDADD += -lidn
-DEFS += -DHAVE_LIBIDN
-endif
+ifeq ($(shell pkg-config --exists 'libidn2 >= 2.0.3' || echo NO),)
+whois_LDADD += $(shell pkg-config --libs libidn2)
+DEFS += -DHAVE_LIBIDN2 $(shell pkg-config --cflags libidn2)
+else ifeq ($(shell pkg-config --exists 'libidn' || echo NO),)
+whois_LDADD += $(shell pkg-config --libs libidn)
+DEFS += -DHAVE_LIBIDN $(shell pkg-config --cflags libidn)
 endif
 
 ifdef HAVE_ICONV

debian/rules

@@ -16,7 +16,7 @@ D := $(CURDIR)/debian/whois
 
 build:
 	dh_testdir
-	$(MAKE) CONFIG_FILE="/etc/whois.conf" HAVE_LIBIDN=1 HAVE_ICONV=1
+	$(MAKE) CONFIG_FILE="/etc/whois.conf" HAVE_ICONV=1
 	touch $@
 
 clean: