Merge remote-tracking branch 'upstream/master'

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diff --git a/jam-files/boost-build/tools/pch.py b/jam-files/boost-build/tools/pch.py
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+# Status: Being ported by Steven Watanabe
+# Base revision: 47077
+#
+# Copyright (c) 2005 Reece H. Dunn.
+# Copyright 2006 Ilya Sokolov
+# Copyright (c) 2008 Steven Watanabe
+#
+# Use, modification and distribution is subject to the Boost Software
+# License Version 1.0. (See accompanying file LICENSE_1_0.txt or
+# http://www.boost.org/LICENSE_1_0.txt)
+
+##### Using Precompiled Headers (Quick Guide) #####
+#
+# Make precompiled mypch.hpp:
+#
+#    import pch ;
+#
+#    cpp-pch mypch
+#      : # sources
+#        mypch.hpp
+#      : # requiremnts
+#        <toolset>msvc:<source>mypch.cpp
+#      ;
+#
+# Add cpp-pch to sources:
+#
+#    exe hello
+#      : main.cpp hello.cpp mypch
+#      ;
+
+from b2.build import type, feature, generators
+
+type.register('PCH', ['pch'])
+type.register('C_PCH', [], 'PCH')
+type.register('CPP_PCH', [], 'PCH')
+
+# Control precompiled header (PCH) generation.
+feature.feature('pch',
+                ['on', 'off'],
+                ['propagated'])
+
+feature.feature('pch-header', [], ['free', 'dependency'])
+feature.feature('pch-file', [], ['free', 'dependency'])
+
+class PchGenerator(generators.Generator):
+    """
+        Base PCH generator. The 'run' method has the logic to prevent this generator
+        from being run unless it's being used for a top-level PCH target.
+    """
+    def action_class(self):
+        return 'compile-action'
+
+    def run(self, project, name, prop_set, sources):
+        if not name:
+            # Unless this generator is invoked as the top-most generator for a
+            # main target, fail. This allows using 'H' type as input type for
+            # this generator, while preventing Boost.Build to try this generator
+            # when not explicitly asked for.
+            #
+            # One bad example is msvc, where pch generator produces both PCH
+            # target and OBJ target, so if there's any header generated (like by
+            # bison, or by msidl), we'd try to use pch generator to get OBJ from
+            # that H, which is completely wrong. By restricting this generator
+            # only to pch main target, such problem is solved.
+            pass
+        else:
+            r = self.run_pch(project, name,
+                 prop_set.add_raw('<define>BOOST_BUILD_PCH_ENABLED'),
+                 sources)
+            return generators.add_usage_requirements(
+                r, ['<define>BOOST_BUILD_PCH_ENABLED'])
+
+    # This rule must be overridden by the derived classes.
+    def run_pch(self, project, name, prop_set, sources):
+        pass
+
+#FIXME: dummy-generator in builtins.jam needs to be ported.
+# NOTE: requirements are empty, default pch generator can be applied when
+# pch=off.
+###generators.register(
+###    [ new dummy-generator pch.default-c-pch-generator   : :   C_PCH ] ;
+###generators.register
+###    [ new dummy-generator pch.default-cpp-pch-generator : : CPP_PCH ] ;