src/sage/features/info.py: new feature for GNU Info

Before we can replace our hand-rolled parser for Singular's info file,
we need to be able to detect the "info" program from the GNU Info
package. The goal is to politely disable access to Singular's info if
GNU Info is unavailable. This avoids a hard dependency on GNU Info in
the sage library.

src/sage/features/info.py

@@ -0,0 +1,31 @@
+# sage_setup: distribution = sagemath-environment
+r"""
+Feature for testing the presence of ``info``, from GNU Info
+"""
+
+from . import Executable
+
+class Info(Executable):
+    r"""
+    A :class:`~sage.features.Feature` describing the presence of :ref:`info <spkg_info>`.
+
+    EXAMPLES::
+
+        sage: from sage.features.info import Info
+        sage: Info().is_present()  # needs info
+        FeatureTestResult('info', True)
+
+    """
+    def __init__(self):
+        r"""
+        TESTS::
+
+            sage: from sage.features.info import Info
+            sage: isinstance(Info(), Info)
+            True
+        """
+        Executable.__init__(self, 'info', executable='info',
+                            spkg='info', type='standard')
+
+def all_features():
+    return [Info()]