Added compilation warning about missing c++ compiler

If the CXX environement variable is not set, then the build must stop
and the user be notified.

SConstruct

@@ -6,6 +6,7 @@ Works on all UNIX-like operating systems.
 
 import os
 import sys
+import SCons.Errors
 
 from fnmatch import fnmatch
 
@@ -80,11 +81,15 @@ class FreelanEnvironment(Environment):
             self.install_prefix = self.prefix
             self.bin_install_prefix = self.bin_prefix
 
-        if os.path.basename(self['CXX']) == 'clang++':
-            self.Append(CXXFLAGS=['-Qunused-arguments'])
-            self.Append(CXXFLAGS=['-fcolor-diagnostics'])
-        elif os.path.basename(self['CXX']).startswith('g++'):
-            self.Append(CXXFLAGS=['-Wno-missing-field-initializers'])
+        if self['CXX'] is None:
+          raise SCons.Errors.BuildError(
+            errstr="CXX environment variable not set. Are you missing a C++ compiler?");
+        else:
+          if os.path.basename(self['CXX']) == 'clang++':
+              self.Append(CXXFLAGS=['-Qunused-arguments'])
+              self.Append(CXXFLAGS=['-fcolor-diagnostics'])
+          elif os.path.basename(self['CXX']).startswith('g++'):
+              self.Append(CXXFLAGS=['-Wno-missing-field-initializers'])
 
         self.Append(CXXFLAGS=['--std=c++11'])
         self.Append(CXXFLAGS=['-Wall'])