Add support for unnamed projects

Sometimes (e.g., in bpkg configuration) we don't have a project name.
In fact, it is not really a project; it can never be referenced in an
import directive.

So we now have a notion of an unnamed project. Such a project should
still have the 'project' variable set first thing in bootstrap.build
but its value should be empty.

Note that we can still amalgamate such projects just liked named ones.

1 files changed, 3 insertions, 0 deletions

diff --git a/tests/amalgam/unnamed/buildfile b/tests/amalgam/unnamed/buildfile
new file mode 100644
index 0000000..5aa12c3
--- /dev/null
+++ b/tests/amalgam/unnamed/buildfile
@@ -0,0 +1,3 @@
+.: sub/
+include sub/
+print $subprojects