Consider replacing/supplementing `common.hpp` with a meson configuration file

[arcondello]

Something like

# Set up the configuration file
conf = configuration_data()
if cpp.has_type('ssize_t', prefix: '#include <sys/types.h>')
    conf.set('HAS_SSIZE_T', 1)
endif
configure_file(
    input: 'dwave/optimization/include/dwave-optimization/config.hpp.in',
    output: 'config.hpp',
    configuration: conf,
)

with a

// config.hpp.in
#mesondefine HAS_SSIZE_T

which we can then include.

I think it may be possible to replace common.hpp entirely with this, but needs more thought/experimentation.