SCons updates

SCons updates
* Blender static now links. By default this option is disabled on all
  platforms. Simply set the option in config.opts to 'true'.
* Added the following flags to config.opts:
  - HOST_CC.    This is the C compiler for the host platform. This value is the
                same as TARGET_CC when not cross compiling.
  - HOST_CXX.   This is the C++ compiler for the host platform. This value is
                the same as TARGET_CXX when not cross compiling.
  - TARGET_CC.  This is the C compiler for the target platform.
  - TARGET_CXX. This is the C++ compiler for the target platform.
  - TARGET_AR.  This is the linker command for linking libraries.
  - PATH        This is the standard search path
  All SConscript files have been updated to reflect these changes. Now it's
  possible to change only the root SConstruct file, and all compiler specific
  variables are passed automatically to all SConscript files. Of course, this
  does not apply to makesdna because there the host and target platform is
  different from all other libraries.
  To pass a variable that applies to all platforms, all we now have to do is
  set the correct value in library_env

Note: as usual, to get the latest options in the config.opts file, first
      remove your version.