Fix the build with GCC 6.

While here, drop USE_GCC=yes, as the port was verified to build with clang on
both HEAD-amd64 and 10.3-i386.

PR:		219288
Approved by:	maintainer timeout (amdmi3, 18 days)

math/drgeo/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	drgeo
 PORTVERSION=	1.1.0
-PORTREVISION=	19
+PORTREVISION=	20
 CATEGORIES=	math
 MASTER_SITES=	SF/ofset/${PORTNAME}/${PORTVERSION}
 
@@ -17,7 +17,6 @@ USES=		gettext gmake pkgconfig pathfix
 USE_GNOME=	gnomeprefix intlhack libglade2
 CPPFLAGS+=	-I${LOCALBASE}/include
 LDFLAGS+=	-L${LOCALBASE}/lib
-USE_GCC=	yes # segfaults when built with clang
 
 PORTDATA=	*
 

math/drgeo/files/patch-geo_drgeo__figure.cc

@@ -0,0 +1,17 @@
+Fix the build with GCC >= 6.
+
+drgeo_figure.cc: At global scope:
+drgeo_figure.cc:56:1: error: '<anonymous struct> drgeoDialogData', declared using anonymous type, is used but never defined [-fpermissive]
+ drgeoDialogData;
+  ^~~~~~~~~~~~~~~
+--- geo/drgeo_figure.cc.orig	2017-07-25 16:56:59 UTC
++++ geo/drgeo_figure.cc
+@@ -48,7 +48,7 @@
+ #include "drgeo_dialog.h"
+ #include "traite.h"
+ 
+-extern struct
++struct
+ {
+   drgeoPoint mouse;
+   drgeoFigure *figure;