message: depends: boost: Split target-os from toolset
  Previously, we specified the target-os in the toolset (and sometimes
  used the wrong command line flags), now we have a clear separation,
  which is favored by ./bootstrap.sh and ./b2.
  This means that all supported OSes will specify the correct target-os=
  and toolset= on the command line.
diff: @@ -9,9 +9,9 @@ $(package)_config_opts_release=variant=release
  $(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
  -$(package)_config_opts_darwin=--toolset=clang-darwin runtime-link=shared
  -$(package)_config_opts_mingw32=binary-format=pe target-os=windows threadapi=win32 runtime-link=static
  +$(package)_config_opts_linux=target-os=linux threadapi=pthread runtime-link=shared
  +$(package)_config_opts_darwin=target-os=darwin runtime-link=shared
  +$(package)_config_opts_mingw32=target-os=windows binary-format=pe 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
  @@ -20,8 +20,8 @@ $(package)_config_opts_aarch64_android=address-model=64
  $(package)_config_opts_x86_64_android=address-model=64
  $(package)_config_opts_armv7a_android=address-model=32
  $(package)_toolset_$(host_os)=gcc
  +$(package)_toolset_darwin=clang
  $(package)_archiver_$(host_os)=$($(package)_ar)
  -$(package)_toolset_darwin=clang-darwin
  $(package)_config_libraries=filesystem,system,thread,test
  $(package)_cxxflags=-std=c++11 -fvisibility=hidden
  $(package)_cxxflags_linux=-fPIC
  @@ -37,9 +37,9 @@ define $(package)_config_cmds
  endef
  define $(package)_build_cmds
  - ./b2 -d2 -j2 -d1 --prefix=$($(package)_staging_prefix_dir) $($(package)_config_opts) stage
  + ./b2 -d2 -j2 -d1 --prefix=$($(package)_staging_prefix_dir) $($(package)_config_opts) toolset=$($(package)_toolset_$(host_os)) stage
  endef
  define $(package)_stage_cmds
  - ./b2 -d0 -j4 --prefix=$($(package)_staging_prefix_dir) $($(package)_config_opts) install
  + ./b2 -d0 -j4 --prefix=$($(package)_staging_prefix_dir) $($(package)_config_opts) toolset=$($(package)_toolset_$(host_os)) install
  endef
  