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

By default orc is configured with `--disable-gtk-doc'. However, despite that it
will checks several `gtkdoc-*' tools anyway ending in an 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 <martin> via PR pkg/52632.

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/devel/orc/Makefile,v
retrieving revision 1.11
diff -u -p -r1.11 Makefile
--- Makefile	20 Jul 2017 13:18:42 -0000	1.11
+++ Makefile	21 Oct 2017 10:39:00 -0000
@@ -14,7 +14,16 @@ USE_LIBTOOL=		yes
 USE_TOOLS=		gmake
 GNU_CONFIGURE=		yes
 PKGCONFIG_OVERRIDE+=	orc.pc.in
+
+# 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=
+
 TEST_TARGET=		check
 
+
 .include "../../mk/pthread.buildlink3.mk"
 .include "../../mk/bsd.pkg.mk"