Add support for modular build structure. (#111)

This is part of the effort to make the Boost libraries "modular" for build and consumption. See https://lists.boost.org/Archives/boost/2024/01/255704.php and https://github.com/grafikrobot/boost-b2-modular/blob/b2-modular/README.adoc for more information.

This PR depends on the following other PRs being merged to both develop and master branches of the respective repos:

- https://github.com/boostorg/boost/pull/854

build.jam

@@ -0,0 +1,35 @@
+# Copyright René Ferdinand Rivera Morell 2023-2024
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at
+# http://www.boost.org/LICENSE_1_0.txt)
+
+require-b2 5.2 ;
+
+constant boost_dependencies :
+    /boost/assert//boost_assert
+    /boost/config//boost_config
+    /boost/container_hash//boost_container_hash
+    /boost/core//boost_core
+    /boost/detail//boost_detail
+    /boost/integer//boost_integer
+    /boost/mpl//boost_mpl
+    /boost/preprocessor//boost_preprocessor
+    /boost/static_assert//boost_static_assert
+    /boost/throw_exception//boost_throw_exception
+    /boost/type_index//boost_type_index
+    /boost/type_traits//boost_type_traits
+    /boost/utility//boost_utility ;
+
+project /boost/variant
+    : common-requirements
+        <include>include
+    ;
+
+explicit
+    [ alias boost_variant : : : : <library>$(boost_dependencies) ]
+    [ alias all : boost_variant test ]
+    ;
+
+call-if : boost-library variant
+    ;
+

test/Jamfile.v2

@@ -8,14 +8,17 @@
 # http://www.boost.org/LICENSE_1_0.txt)
 #
 
+require-b2 5.0.1 ;
+import-search /boost/config/checks ;
+import config : requires ;
 import testing ;
-import ../../config/checks/config : requires ;
 
 local below_cpp14 = 98 03 0x 11 ;
 local since_cpp20 = 2a 20 latest ;
 
 project
     : requirements
+        <library>/boost/variant//boost_variant
         [ requires cxx11_rvalue_references ]
         <toolset>msvc:<asynch-exceptions>on
     ;
@@ -30,7 +33,7 @@ test-suite variant
     [ run test6.cpp : : : : variant_test6 ]
     [ run test7.cpp : : : : variant_test7 ]
     [ run test8.cpp : : : : variant_test8 ]
-    [ run test9.cpp : : : : variant_test9 ]
+    [ run test9.cpp : : : <library>/boost/fusion//boost_fusion : variant_test9 ]
     [ run recursive_variant_test.cpp ]
     [ run variant_reference_test.cpp ]
     [ run variant_comparison_test.cpp ]
@@ -39,7 +42,7 @@ test-suite variant
     [ run variant_get_test.cpp ]
     [ compile-fail variant_rvalue_get_with_ampersand_test.cpp ]
     [ compile-fail no_rvalue_to_nonconst_visitation.cpp ]
-    [ compile fusion_interop.cpp ]
+    [ compile fusion_interop.cpp : <library>/boost/fusion//boost_fusion ]
     [ run variant_polymorphic_get_test.cpp ]
     [ run variant_multivisit_test.cpp ]
     [ run hash_variant_test.cpp ]
@@ -58,13 +61,13 @@ test-suite variant
     [ run recursive_variant_test.cpp : : : <rtti>off <define>BOOST_NO_RTTI <define>BOOST_NO_TYPEID : variant_no_rtti_test ]
     [ run hash_recursive_variant_test.cpp ]
     [ run variant_swap_test.cpp ]
-    [ run auto_visitors.cpp ]
+    [ run auto_visitors.cpp : : : <library>/boost/lexical_cast//boost_lexical_cast ]
     [ run issue42.cpp ]
-    [ run issue53.cpp ]
+    [ run issue53.cpp : : : <library>/boost/thread//boost_thread ]
     [ run overload_selection.cpp : : : "<cxxstd>$(since_cpp20)"\:<build>no ]
-    [ run recursive_wrapper_move_test.cpp ]
+    [ run recursive_wrapper_move_test.cpp : : : <library>/boost/array//boost_array ]
     [ run variant_over_joint_view_test.cpp ]
-    [ run const_ref_apply_visitor.cpp ]
+    [ run const_ref_apply_visitor.cpp : : : <library>/boost/lexical_cast//boost_lexical_cast ]
-   ; 
+   ;