Configuration issues with clang++ on mac silicon

[BoPeng]

When compiling simuPOP from source,

#define HAVE_DECL_ISFINITE 1

is needed for line 1099 of config.h.

Then, we have error like

src/utility.h:2422:9: error: no member named '_Bit_type' in namespace 'std'

We may need another set of macro definitions to support this particular configuration.