Implement assert directive

The grammar is as follows:

assert <expression> [<description>]
assert! <expression> [<description>]

The expression must evaluate to 'true' or 'false', just like in if-else.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/directive/buildfile b/tests/directive/buildfile
new file mode 100644
index 0000000..e9e946e
--- /dev/null
+++ b/tests/directive/buildfile
@@ -0,0 +1,7 @@
+# file      : tests/directive/buildfile
+# copyright : Copyright (c) 2014-2016 Code Synthesis Ltd
+# license   : MIT; see accompanying LICENSE file
+
+./: test{assert} file{common.test}
+
+test{*}: test = $effect($build.path)