libidn2: Fix installation if gtk-doc package is already installed

By default libidn2 is configured with `--disable-gtk-doc'. However,
despite that, it will checks several `gtkdoc-*' tools anyway, ending
in a failure during the installation phase because only a couple
of them are available.

Add a kludge via CONFIGURE_ENV to avoid picking up gtkdoc-* tools.

Pointed out by <scole> via PR pkg/52667.
Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/devel/libidn2/Makefile,v
retrieving revision 1.4
diff -u -p -r1.4 Makefile
--- Makefile	30 Aug 2017 15:22:30 -0000	1.4
+++ Makefile	28 Oct 2017 21:57:02 -0000
@@ -15,6 +15,13 @@ GNU_CONFIGURE=		yes
 PKGCONFIG_OVERRIDE+=	libidn2.pc.in
 INFO_FILES=		yes
 
+# XXX: Kludge to avoid uncoditionally pick up some gtkdoc-* tools if gtk-doc
+# XXX: package is installed and then fails in the installation phase.
+CONFIGURE_ENV+=		ac_cv_prog_GTKDOC_CHECK= \
+			ac_cv_path_GTKDOC_CHECK_PATH= \
+			ac_cv_path_GTKDOC_REBASE= \
+			ac_cv_path_GTKDOC_MKPDF=
+
 .include "../../converters/libiconv/buildlink3.mk"
 .include "../../devel/gettext-lib/buildlink3.mk"
 .include "../../textproc/libunistring/buildlink3.mk"