Make a real Theano flags cxx of gxx_avail.

doc/library/config.txt

@@ -363,6 +363,21 @@ import theano and print the config variable, as in:
     Extra parameters to pass to gcc when compiling.  Extra include paths,
     library paths, configuration options, etc.
 
+.. attribute:: config.cxx
+
+
+    Default: 'g++' if g++ is present. '' Otherwise.
+
+    Tell the c++ compiler to use. If empty, don't compile c++ code.
+    We automatically detect if g++ is present and disable it if not
+    present.
+
+    We print a warning if we detect that g++ is not present. It is
+    recommended to run with c++ compilation as Theano will be much
+    slower otherwise.
+
+    Currently only g++ is supported, but supporting others is easy.
+
 .. attribute:: optimizer_excluding
 
     Default: ""

theano/configdefaults.py

@@ -87,7 +87,8 @@ AddConfigVar('mode',
                 'FAST_COMPILE', 'PROFILE_MODE', 'DEBUG_MODE'),
         in_c_key=False)
 
-gxx_avail = True
+enum = EnumStr("g++", "")
+
 # Test whether or not g++ is present: disable C code if it is not.
 # Using the dummy file descriptor below is a workaround for a crash experienced
 # in an unusual Python 2.4.4 Windows environment with the default stdin=None.
@@ -113,11 +114,19 @@ except OSError:
             'optimized C-implementations (for both CPU and GPU) and will '
             'default to Python implementations. Performance will be severely '
             'degraded.')
-    gxx_avail = False
+    enum = EnumStr("")
 
 del dummy_stdin
+AddConfigVar('cxx',
+             "The c++ compiler to use. Currently only g++ is"
+             " supported. But supporting more is easy if someone want this."
+             "If it is empty, we don't compile c++ code.",
+             enum,
+             in_c_key=False)
+del enum
 
-#Keep the default optimizer the same as the one for the mode FAST_RUN
+
+#Keep the default value the same as the one for the mode FAST_RUN
 AddConfigVar('allow_gc',
              "Do we default to delete intermediate results during Theano"
              " function calls? Doing so lowers the memory requirement, but"