Add LIB_CXX so that C++ libraries will use CXX to link.

This fixes C++ libraries not implicitly linking in libc++.  This is
generally not an issue because the final linking with the compiled binary
will involve CXX via PROG_CXX or other means.  It is however
inconsistent with libraries implicitly linking in libc and problematic
for trying to build libraries with '-z defs' to ensure all direct
dependencies are linked in.

libatf-c++ is currently the only consumer of this new feature.

Differential Revision:	https://reviews.freebsd.org/D2039
Reviewed by:	imp
Discussed with:	bapt
MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division

Makefile.inc1

@@ -1685,6 +1685,10 @@ gnu/lib/libstdc++__L: lib/msun__L
 gnu/lib/libsupc++__L: gnu/lib/libstdc++__L
 .endif
 
+.if ${MK_LIBCPLUSPLUS} != "no"
+_prebuild_libs+= lib/libc++
+.endif
+
 lib/libgeom__L: lib/libexpat__L
 
 .if ${MK_LIBTHR} != "no"

lib/atf/libatf-c++/Makefile

@@ -28,7 +28,7 @@
 .include <src.opts.mk>
 .include <bsd.init.mk>
 
-LIB=		atf-c++
+LIB_CXX=	atf-c++
 PRIVATELIB=	true
 SHLIB_MAJOR=	2
 

share/mk/bsd.lib.mk

@@ -4,6 +4,13 @@
 
 .include <bsd.init.mk>
 
+.if defined(LIB_CXX)
+LIB=	${LIB_CXX}
+_LD=	${CXX}
+.else
+_LD=	${CC}
+.endif
+
 # Set up the variables controlling shared libraries.  After this section,
 # SHLIB_NAME will be defined only if we are to create a shared library.
 # SHLIB_LINK will be defined only if we are to create a link to it.
@@ -217,7 +224,7 @@ ${SHLIB_NAME_FULL}: ${SOBJS}
 .if defined(SHLIB_LINK)
 	@${INSTALL_SYMLINK} ${SHLIB_NAME} ${SHLIB_LINK}
 .endif
-	${CC} ${LDFLAGS} ${SSP_CFLAGS} ${SOLINKOPTS} \
+	${_LD} ${LDFLAGS} ${SSP_CFLAGS} ${SOLINKOPTS} \
 	    -o ${.TARGET} -Wl,-soname,${SONAME} \
 	    `NM='${NM}' lorder ${SOBJS} | tsort -q` ${LDADD}
 .if ${MK_CTF} != "no"