Added illumos macro for int8_t parse_value override.

[jMichaelA]

illumos' implementation of stdint.h treats chars as int8_t. So when I was trying to compile this on an illumos distro I was getting the following compilation error:

/usr/local/src/geos-3.12.0/util/geosop/cxxopts.hpp:793:10: error: redefinition of 'void cxxopts::values::parse_value(const string&, char&)'
  793 |     void parse_value(const std::string& text, char& c)
      |          ^~~~~~~~~~~
/usr/local/src/geos-3.12.0/util/geosop/cxxopts.hpp:682:5: note: 'void cxxopts::values::parse_value(const string&, int8_t& ' previously defined here
  682 |     parse_value(const std::string& text, int8_t& value)
      |     ^~~~~~~~~~~

So this change will only add one of the functions if it's an illumOS system.

[pramsey]

Thanks!