Introduce 'toolset requirements'

[SVN r36076]

src/build/targets.jam

@@ -82,6 +82,7 @@ import path ;
 import set ;
 import assert ;
 import indirect ;
+import toolset ;
 
 # Base class for all abstract targets.
 class abstract-target 
@@ -1408,6 +1409,8 @@ rule main-target-requirements (
   : project       # Project where the main target is to be declared
       )
 {
+    specification += [ toolset.requirements ] ;
+    
     local requirements = [ property-set.refine-from-user-input 
         [ $(project).get requirements ] : $(specification) :
         [ $(project).project-module ] : [ $(project).get location ] ] ;

src/build/toolset.jam

@@ -465,7 +465,23 @@ rule inherit-rules ( toolset : base )
     IMPORT $(toolset) : $(rules) : : $(toolset).$(rules) ;
 }
 
+# Return the list of global 'toolset requirements'.
+# Those requirements will be automatically added to
+# the requirements of any main target.
+rule requirements ( )
+{
+    return $(.requirements) ;
+}
 
+# Adds elements to the list of global 'toolset requirements'.
+# The requirements will be automatically added to the requirements
+# for all main targets, as if they were specified literally.
+# For best results, all requirements added should be conditional or
+# indirect conditional.
+rule add-requirements ( requirements * )
+{
+    .requirements += $(requirements) ;
+}
 
 local rule __test__ ( )
 {