Fix Clang build and incorrect _WIN32 macros

Sundaram Ramaswamyrequested to merge
legends2k/highlight:fix_win32_macro into master

Clang complains of unknown getpid. Include POSIX's unistd.h on *nix platforms but on Windows include process.h as MSVC only has latter.

Though we only use MinGW on Windows, this is a safer option since MinGW has both headers.

_WIN32 is the official macro 1 while WIN32 is incorrectly used in code. This was fixed partly in !136 (merged) but lot of occurances were missed.

Edited by Sundaram Ramaswamy

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