Fix viable-source-types-for-generator-real looping thinko.

The loop over 'source-types' was meant to terminate if we ever get
'*'. However, it was for loop, and assinging empty list to
source-types had no effect. As result, the function would
sometimes return a list containing '*' and other types, which
other code did not expect and mishandled.


[SVN r55712]

src/build/generators.jam

@@ -952,8 +952,10 @@ rule viable-source-types-for-generator-real ( generator )
     else
     {
         local result ;
-        for local s in $(source-types)
+        while $(source-types)
         {
+            local s = $(source-types[1]) ;
+            source-types = $(source-types[2-]) ;
             local viable-sources = [ generators.viable-source-types $(s) ] ;
             if $(viable-sources) = *
             {