miscellaneous fixes

[sezero]

A few minor fixes, and a cmake update/fix:

• adpcm-dns.c: fix unused parameter warning for generate_dns_values()
• adpcm-xq.c: fix -Wold-style-definition for main()
• fix build in C89 mode
• adpcm-lib.c: silence 'may be used uninitialized' warning for best_index
• cmake fixes / updates:
  • fix library target name and sources
  • link adpcm-xq exe to adpcm-lib
  • link to libm if available, for floor(), log10(), etc.
  • enable -Wall