Add a c++/v1/tr1 include directory containing symlinks to all of the standard

headrs.

Lots of third-party code expects to find C++03 headers under tr1 because that's
where GNU decided to hide them.  This should fix ports that expect them there.

MFC after:	1 week

etc/mtree/BSD.include.dist

@@ -83,6 +83,8 @@
         v1
             ext
             ..
+            tr1
+            ..
         ..
     ..
     cam

lib/libc++/Makefile

@@ -165,6 +165,7 @@ RT_HEADERS=	cxxabi.h\
 
 .for hdr in ${STD_HEADERS}
 STD+=		${HDRDIR}/${hdr}
+INCSLINKS+=	${CXXINCLUDEDIR}/${hdr} ${CXXINCLUDEDIR}/tr1/${hdr}
 .endfor
 .for hdr in ${RT_HEADERS}
 STD+=		${LIBCXXRTDIR}/${hdr}