Work on BB5. Mininal version of toolset initialization implemented.

* new/gcc.jam (init): New rule. Allows to specify alternative
    gcc versions.

* new/toolset.jam
    (using): New rule.
    (set-target-variables): Bufgix. I was making indirect call
        via variable with multiple values.

* new/build-request.jam
   (looks-like-implicit-value): New rule.
   (from-command-line): Use the above.
   (convert-command-line-element): Don't validate
      elements with dashes (this probably must be fixed).


[SVN r16468]

tools/build

@@ -1 +1 @@
-Subproject commit 075a9eaf9703a2075b0385703b24e9b57be6bb2e
+Subproject commit a8d2feea847d9dbd53220647d0886739dd3ce245