builddoc: Use '[sphinx_build] builders' with Sphinx < 1.6

In 'c024066', we resolved an issue with pbr and Sphinx 1.6+ and
implicitly started using Sphinx 1.6's support for declaring multiple
builders as part of the setuptools plugin. However, in doing so, we
neglected to provide a migration path for folks who were using the older
option. This meant folks using the '[sphinx_build] builders' option
would see a breaking change in behavior when upgrading to a pbr release
including the above change. Resolve this issue now.

Note that we want to deprecate support for this option in the future,
but doing so is not a backportable change and will be done separately.

Change-Id: Ic2fe67f932ba26b665110ae66431a5359fc50016
Closes-Bug: #1702872
Related-Bug: #1691129

pbr/builddoc.py

@@ -25,6 +25,7 @@ except ImportError:
     import io as cStringIO
 
 try:
+    import sphinx
     from sphinx import apidoc
     from sphinx import application
     from sphinx import setup_command
@@ -42,6 +43,7 @@ except Exception as e:
     raise ImportError(str(e))
 from pbr import git
 from pbr import options
+from pbr import version
 
 
 _rst_template = """%(heading)s
@@ -186,13 +188,21 @@ class LocalBuildDoc(setup_command.BuildDoc):
                         "autodoc_exclude_modules",
                         [None, ""])[1].split()))
 
-        # TODO(stephenfin): Deprecate this functionality once we depend on
+        self.finalize_options()
+
+        is_multibuilder_sphinx = version.SemanticVersion.from_pip_string(
+            sphinx.__version__) >= version.SemanticVersion(1, 6)
+
+        if self.builders != ['html'] and is_multibuilder_sphinx:
+            self.builder = self.builders
+
+        # TODO(stephenfin): Deprecate this functionality once we decide on
         # Sphinx 1.6, which includes a similar feature, in g-r
         # https://github.com/sphinx-doc/sphinx/pull/3476
-        self.finalize_options()
-        if hasattr(self, "builder_target_dirs"):
-            # Sphinx >= 1.6.1
+        if is_multibuilder_sphinx:
+            # Sphinx >= 1.6
             return setup_command.BuildDoc.run(self)
+
         # Sphinx < 1.6
         for builder in self.builders:
             self.builder = builder