Implement support for dependency chains

Now instead of:

./: exe{foo}
exe{foo}: cxx{*}

We can write:

./: exe{foo}: cxx{*}

Or even:

./: exe{foo}: libue{foo}: cxx{*}

This can be combined with prerequisite-specific variables (which naturally
only apply to the last set of prerequisites in the chain):

./: exe{foo}: libue{foo}: bin.whole = false

1 files changed, 9 insertions, 4 deletions

diff --git a/tests/variable/prerequisite-specific/testscript b/tests/variable/prerequisite-specific/testscript
index 1c7e7bd..ef14cfc 100644
--- a/tests/variable/prerequisite-specific/testscript
+++ b/tests/variable/prerequisite-specific/testscript
@@ -62,10 +62,15 @@ EOI
   }
 EOE
 
-: expect-assignment
+: chain
 :
-$* <<EOI 2>>EOE != 0
-dir{x}: dir{a}:
+$* <<EOI 2>>/~%EOE%
+dir{x}: dir{y}: dir{a}: foo = FOO
+dump dir{y}
 EOI
-<stdin>:1:16: error: expected name instead of <newline>
+<stdin>:2:1: dump:
+%  .+/dir\{y/\}: .+:dir\{a/\}:%
+  {
+    foo = FOO
+  }
 EOE