adding rgba conversion convenience functionality

rgba conversion is tough, this request makes it a little easier with:
 * autoconf existence check for endian.h
 * inline.h now:
  * is unix- (not DOS-) encoded
  * includes endian.h
  * defines new struct rgba_t
  * defines new union un_rgba_t
  * defines new function get_rgba that converts 32-bit integer to rgba_t
 * Makefile.am has additional test_endian, unit test for everything above
 * test/endian.cc, source for the unit test