Add libc++experimental.a for std::experimental support

This adds a separate library for supporting std::experimental features.
It is purposefully static, and must be explicitly linked into programs
using -lc++experimental.

PLEASE NOTE: there is NO WARRANTY as to any stability or continuing
existence of the features in the std::experimental parts of the C++
library!

Reviewed by:	ed
Differential Revision: https://reviews.freebsd.org/D10840

lib/Makefile

@@ -156,7 +156,7 @@ _libclang_rt=	libclang_rt
 
 .if ${MK_LIBCPLUSPLUS} != "no"
 _libcxxrt=	libcxxrt
-_libcplusplus=	libc++
+_libcplusplus=	libc++ libc++experimental
 .endif
 
 SUBDIR.${MK_EFI}+=	libefivar

lib/libc++experimental/Makefile

@@ -0,0 +1,29 @@
+# $FreeBSD$
+
+.include <src.opts.mk>
+
+PACKAGE=	clibs
+SRCDIR=		${SRCTOP}/contrib/libc++
+
+LIB=		c++experimental
+NO_PIC=
+MK_PROFILE=	no
+
+.PATH:		${SRCDIR}/src/experimental
+
+SRCS+=		filesystem/directory_iterator.cpp
+SRCS+=		filesystem/operations.cpp
+SRCS+=		filesystem/path.cpp
+SRCS+=		memory_resource.cpp
+
+WARNS?=		0
+CXXFLAGS+=	-isystem ${SRCDIR}/include
+CXXFLAGS+=	-nostdinc++
+CXXFLAGS+=	-nostdlib
+CXXFLAGS+=	-D_LIBCPP_BUILDING_LIBRARY
+CXXFLAGS+=	-DLIBCXXRT
+.if empty(CXXFLAGS:M-std=*)
+CXXFLAGS+=	-std=c++14
+.endif
+
+.include <bsd.lib.mk>

tools/build/mk/OptionalObsoleteFiles.inc

@@ -4512,6 +4512,7 @@ OLD_LIBS+=${DEBUG_LIB32_LIBS}
 OLD_LIBS+=lib/libcxxrt.so.1
 OLD_FILES+=usr/lib/libc++.a
 OLD_FILES+=usr/lib/libc++_p.a
+OLD_FILES+=usr/lib/libc++experimental.a
 OLD_FILES+=usr/lib/libc++.so
 OLD_LIBS+=usr/lib/libc++.so.1
 OLD_FILES+=usr/lib/libcxxrt.a