Properly detect endianness before setting -D__LITTLE_ENDIAN__ (issue #7)

[bigon]

TBH I'm not even sure that the whole "AC_SUBST(STK_CXXFLAGS, "-D__LINUX_ALSA__ -D__UNIX_JACK__")" is even necessary, as this seems only needed during the compilation of stk itself

Anyway, if you feel it necessary, this branch should FTBFS on big-endian architectures