configury: make sure we have a src subdir in VPATH builds.
configury: make sure we have a src subdir in VPATH builds.

* src/src.mk (src/specl): Make a `src` sub-directory if we don't
already have one.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl