Add USE_CASE_MAP_API config

[k-takata]

The case_map API is mainly (only?) used in Ruby.
Make it possible to disable the API.

[codecov-io]

Codecov Report

Merging #125 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #125   +/-   ##
======================================
  Coverage    82.7%   82.7%           
======================================
  Files          62      62           
  Lines       12435   12435           
======================================
  Hits        10284   10284           
  Misses       2151    2151

Impacted Files	Coverage Δ
enc/koi8_r.c	53.84% <ø> (ø)	⬆️
enc/iso_8859_14.c	3.5% <ø> (ø)	⬆️
enc/iso_8859_15.c	20.37% <ø> (ø)	⬆️
enc/iso_8859_9.c	3.84% <ø> (ø)	⬆️
enc/iso_8859_7.c	4.25% <ø> (ø)	⬆️
enc/iso_8859_8.c	0% <ø> (ø)	⬆️
enc/windows_1254.c	19.64% <ø> (ø)	⬆️
enc/euc_jp.c	91.8% <ø> (ø)	⬆️
enc/gb18030.c	13.68% <ø> (ø)	⬆️
enc/utf_32be.c	95.12% <ø> (ø)	⬆️
... and 25 more

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[coveralls]

Coverage remained the same at 80.658% when pulling 80e289d on case_map-config into 8d958db on master.