private/public tests with (optional)

Currently, a library marked (optional) will not build if it's public and has a
private dependency.

test/blackbox-tests/test-cases/private-public-overlap/optional/jbuild

@@ -0,0 +1,12 @@
+(jbuild_version 1)
+
+(library
+ ((name privatelib)
+  (modules (privatelib))))
+
+(library
+ ((name publiclib)
+  (public_name publiclib)
+  (modules (publiclib))
+  (libraries (privatelib))
+  (optional)))

test/blackbox-tests/test-cases/private-public-overlap/run.t

@@ -38,3 +38,7 @@ However, public binaries may accept private dependencies
         ocamlc .publicbin.eobjs/publicbin.{cmi,cmo,cmt}
       ocamlopt .publicbin.eobjs/publicbin.{cmx,o}
       ocamlopt publicbin.exe
+
+Private dependencies shouldn't make the library optional
+  $ $JBUILDER build -j1 --display short --root optional 2>&1 | grep -v Entering
+  [1]