Use array instead of long switch for faster compilation

The long switch in src/hnc2unicode.c just makes compilation slow
uselessly. It does not reduce the size.

Convert the very long switch into a array for faster compilation. And
move array data in src/hnc2unicode.h to src/hnc2unicode.inc.

Add AC_PROG_CC_C99 in configure.ac, because designated array
initializers needs C99.