After r253839, which modifies ld's behaviour to not automatically pull

in needed libraries, change libc++.so into a linker script, so it can
automatically pull in libcxxrt.so.

MFC after:	1 week

lib/libc++/Makefile

@@ -9,6 +9,7 @@ CXXINCLUDEDIR=	${INCLUDEDIR}/c++/v${SHLIB_MAJOR}
 
 LIB=		c++
 SHLIB_MAJOR=	1
+SHLIB_LDSCRIPT=	libc++.ldscript
 
 SRCS+=		algorithm.cpp\
 		bind.cpp\

lib/libc++/libc++.ldscript

@@ -0,0 +1,2 @@
+/* $FreeBSD$ */
+GROUP ( @@SHLIB@@ @@LIBDIR@@/libcxxrt.so )