doctests: use dochtml as default option only when html docs are insta…

…lled

src/bin/sage-runtests

@@ -8,14 +8,15 @@ import sys
 DOT_SAGE = os.environ.get('DOT_SAGE', os.path.join(os.environ.get('HOME'),
                                                    '.sage'))
 SAGE_ROOT = os.environ.get('SAGE_ROOT')
+SAGE_DOC = os.environ.get('SAGE_DOC')
 
 # Override to not pick up user configuration, see Trac #20270
 os.environ['SAGE_STARTUP_FILE'] = os.path.join(DOT_SAGE, 'init-doctests.sage')
 
 
 def _get_optional_defaults():
     """Return the default value for the --optional flag."""
-    optional = ['sage', 'dochtml', 'optional']
+    optional = ['sage', 'optional']
 
     # This should be good-enough to determine if we are in a sage source
     # repository working tree
@@ -24,6 +25,10 @@ def _get_optional_defaults():
             os.path.isfile(os.path.join(SAGE_ROOT, 'sage'))):
         optional.append('build')
 
+    # Check if the html docs are installed
+    if (SAGE_DOC and os.path.isdir(os.path.join(SAGE_DOC, 'html'))):
+        optional.append('dochtml')
+
     return ','.join(optional)