CPP: create platform defines

[certik]

At least: __linux__ and __APPLE__.

[Shaikh-Ubaid]

It seems like gfortran does not support __APPLE__.

% cat examples/expr2.f90 
program test

#ifdef _WIN32
    print *,'Windows'
#endif
#ifdef __linux
    print *,'Linux'
#endif
#ifdef __APPLE__
    print *,'OSX'
#endif

end program
% gfortran examples/expr2.f90 -cpp
% ./a.out 
% cat examples/expr2.cpp 
#include<iostream>

int main() {
    #ifdef _WIN32
        std::cout << "Windows" << std::endl;
    #endif
    #ifdef __linux
        std::cout << "Linux" << std::endl;
    #endif
    #ifdef __APPLE__
        std::cout << "OSX" << std::endl;
    #endif
    return 0;
}
% g++ examples/expr2.cpp          
% ./a.out 
OSX