Fails to build on arm64

[falkTX]

Due to some WDL things, build error being:

thirdparty/WDL/source/WDL/wdltypes.h:203:69: error: narrowing conversion of '-1' from 'int' to 'long unsigned int' [-Wnarrowing]
  203 | typedef char wdl_assert_failed_unsigned_char[((char)-1) > 0 ? -1 : 1];
      |                                                                     ^
thirdparty/WDL/source/WDL/wdltypes.h:203:61: error: size '-1' of array 'wdl_assert_failed_unsigned_char' is negative
  203 | typedef char wdl_assert_failed_unsigned_char[((char)-1) > 0 ? -1 : 1];
      |                                              ~~~~~~~~~~~~~~~^~~~~~~~

[falkTX]

The same error happens on armhf too

[jpcima]

See: https://www.askjf.com/?q=5516s

There seems 2 ways to go about this problem; one is -fsigned-char, the other is -DWDL_ALLOW_UNSIGNED_DEFAULT_CHAR.

Judging from Justin's comment, the former option is preferable.