Index: head/lang/gcc5/Makefile.DragonFly
===================================================================
--- head/lang/gcc5/Makefile.DragonFly	(revision 381534)
+++ head/lang/gcc5/Makefile.DragonFly	(revision 381535)
@@ -1,3 +1,11 @@
 # $FreeBSD$
 
 OPTIONS_EXCLUDE= JAVA
+
+# The following modification to CXXFLAGS is needed in the case of DragonFly
+# 4.1+ building lang/gcc5 with the gcc50 base compiler when the system
+# libraries were built by the gcc47 base compiler.  The sys.mk file
+# automatically adds -D_GLIBCXX_USE_CXX11_ABI=0 to CXXFLAGS in that case,
+# which will break the build.
+
+CXXFLAGS:=	${CXXFLAGS:N-D_GLIBCXX_USE_CXX11_ABI*}