depends: fix boost mac cross build with clang 9+

The ancient "darwin-4.9.1" profile has long been used to match against
clang, which prior to version 9, reported 4.9.1 as its version when
invoking "clang++ -dumpversion". Presumably this was a historical
compatibility quirk related to Apple's switch from gcc to clang.

This was "fixed" in clang 9.0, so that -dumpversion reports the real
version. Unfortunately that had the side-effect of breaking the
(brittle) boost compiler detection.

Move to the seemingly more-correct "clang-darwin" profile, which passes
the checks and builds correctly.

Also switch to using ar rather than libtool for archiving, as it's what
the clang-darwin profile expects to be using.

Note that because this is using a different profile, some of the final
command-line arguments end up changing. The changes look sane at a
glance.

depends/packages/boost.mk

@@ -10,15 +10,14 @@ $(package)_config_opts_debug=variant=debug
 $(package)_config_opts=--layout=tagged --build-type=complete --user-config=user-config.jam
 $(package)_config_opts+=threading=multi link=static -sNO_BZIP2=1 -sNO_ZLIB=1
 $(package)_config_opts_linux=threadapi=pthread runtime-link=shared
-$(package)_config_opts_darwin=--toolset=darwin-4.2.1 runtime-link=shared
+$(package)_config_opts_darwin=--toolset=clang-darwin runtime-link=shared
 $(package)_config_opts_mingw32=binary-format=pe target-os=windows threadapi=win32 runtime-link=static
 $(package)_config_opts_x86_64_mingw32=address-model=64
 $(package)_config_opts_i686_mingw32=address-model=32
 $(package)_config_opts_i686_linux=address-model=32 architecture=x86
 $(package)_toolset_$(host_os)=gcc
 $(package)_archiver_$(host_os)=$($(package)_ar)
-$(package)_toolset_darwin=darwin
-$(package)_archiver_darwin=$($(package)_libtool)
+$(package)_toolset_darwin=clang-darwin
 $(package)_config_libraries=chrono,filesystem,system,thread,test
 $(package)_cxxflags=-std=c++11 -fvisibility=hidden
 $(package)_cxxflags_linux=-fPIC