Build: Make OpenMP usage opt-out

Can be disabled using "scons ... enable_openmp=false"
Note that on some systems, like my Core2Duo with HyperThreading on Arch Linux,
one gets better performance when explicitly setting the number of threads to
the number of physical cores. For instance: export OMP_NUM_THREADS=2

SConstruct

@@ -34,7 +34,7 @@ opts.Add(BoolVariable('enable_gegl', 'enable GEGL based code in build', False))
 opts.Add(BoolVariable('enable_introspection', 'enable GObject introspection support', False))
 opts.Add(BoolVariable('enable_docs', 'enable documentation build', False))
 opts.Add(BoolVariable('enable_gperftools', 'enable gperftools in build, for profiling', False))
-opts.Add(BoolVariable('enable_openmp', 'enable OpenMP for libmypaint', False))
+opts.Add(BoolVariable('enable_openmp', 'enable OpenMP for multithreaded processing (on by default)', True))
 opts.Add('python_binary', 'python executable to build for', default_python_binary)
 opts.Add('python_config', 'python-config to used', default_python_config)