COMP: use standard compiler macros when distinguishing the OS

- replace (darwin) with (__APPLE__)
- replace (solarisGcc) with (__sun__ && __GNUC__)
- instead of 'darwin' -> '__APPLE'

- cease with passing a -D$(WM_ARCH) define since this adds no useful
  additional information and isn't used anywhere.

Reference
http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system

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COMP: Extend size disambiguation on long (#1238)