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, 5 insertions, 0 deletions

diff --git a/tests/dependency/chain/buildfile b/tests/dependency/chain/buildfile
new file mode 100644
index 0000000..a27681e
--- /dev/null
+++ b/tests/dependency/chain/buildfile
@@ -0,0 +1,5 @@
+# file      : tests/dependency/chain/buildfile
+# copyright : Copyright (c) 2014-2018 Code Synthesis Ltd
+# license   : MIT; see accompanying LICENSE file
+
+./: testscript $b