Before making the c-linker the default one, check whether gcc is present or not.

theano/configdefaults.py

 import os
+import subprocess
+import logging
+
 from theano.configparser import TheanoConfigParser, AddConfigVar, EnumStr, StrParam, IntParam, FloatParam, BoolParam
 
+_logger = logging.getLogger('theano.configdefaults')
+def warning(*msg):
+    _logger.warning('WARNING theano.configdefaults: '+' '.join(msg))
+
 config = TheanoConfigParser()
 
 AddConfigVar('floatX',
@@ -24,10 +31,22 @@ AddConfigVar('mode',
         "Default compilation mode",
         EnumStr('Mode', 'ProfileMode', 'DebugMode', 'FAST_RUN', 'FAST_COMPILE', 'PROFILE_MODE', 'DEBUG_MODE'))
 
-#Keep the default linker the same as the one for the mode FAST_RUN
-AddConfigVar('linker',
-        "Default linker. If not None, will use this linker with the Mode object(not ProfileMode or DebugMode)",
-        EnumStr('c|py', 'py', 'c', 'c|py_nogc', 'c&py'))
+# Test whether or not gcc is present: disable C code if it is not
+try:
+    subprocess.Popen('gcc', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    # Keep the default linker the same as the one for the mode FAST_RUN
+    AddConfigVar('linker',
+            "Default linker. If not None, will use this linker with the Mode "+
+            "object(not ProfileMode or DebugMode)",
+            EnumStr('c|py', 'py', 'c', 'c|py_nogc', 'c&py'))
+except OSError:
+    # gcc is not present, linker should default to python only
+    AddConfigVar('linker',
+            "Default linker. If not None, will use this linker with the Mode object(not ProfileMode or DebugMode)",
+            EnumStr('py', 'c|py', 'c', 'c|py_nogc', 'c&py'))
+    warning('GCC not detected ! Theano will be unable to execute optimized '+
+            'C-implementations (for both CPU and GPU) and will default to '+
+            'Python implementations. Performance will be severely degraded.')
 
 #Keep the default optimizer the same as the one for the mode FAST_RUN
 AddConfigVar('optimizer',