build, utils: Adjust how the floating point tolerance is defined
A subsequent commit will switch to using G_APPROX_VALUE instead of photos_utils_equal_double. Since G_APPROX_VALUE requires specifying the tolerance value, it would be convenient to have it defined somewhere that's already accessible to every source file to avoid having to #include a separate header to compare floating point values. Every source file already #includes config.h, so that's the natural choice.
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