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Edit #2

So after looking into how the bits might be stored on different systems, I finally realized that I might be able to just use a single bit to test for endianness. It looks to be much less error-prone than using an array and I still get the correct answer on my Windows box.

enum hl_endianness : uint32_t {
    HL_LITTLE_ENDIAN   = 0x00000001,
    HL_BIG_ENDIAN      = 0x01000000,
    HL_PDP_ENDIAN      = 0x00010000,
    HL_UNKNOWN_ENDIAN  = 0xFFFFFFFF
};

/**
 * A simple function that can be used to help determine a program's endianness
 * at compile-time.
 */
constexpr hl_endianness getEndianOrder() {
    return
        ((1 & 0xFFFFFFFF) == HL_LITTLE_ENDIAN)
            ? HL_LITTLE_ENDIAN
            : ((1 & 0xFFFFFFFF) == HL_BIG_ENDIAN)
                ? HL_BIG_ENDIAN
                : ((1 & 0xFFFFFFFF) == HL_PDP_ENDIAN)
                    ? HL_PDP_ENDIAN
                    : HL_UNKNOWN_ENDIAN;
}

#define HL_ENDIANNESS getEndianOrder()

Edit:

I can try dereferencing the pointer offset to the endianValues array but I'm still not sure if it will end up defaulting to 0 (the first explicitly set value in the array).

constexpr hl_endianness getEndianOrder() {
    return
        (0x00 == *endianValues)           // If Little Endian Byte Order,
            ? HL_LITTLE_ENDIAN              // return 0 for little endian.
            : (0x03 == *endianValues)     // Else if Big Endian Byte Order,
                ? HL_BIG_ENDIAN             // return 1 for big endian.
                : (0x02 == *endianValues) // Else if PDP Endian Byte Order,
                    ? HL_PDP_ENDIAN         // return 2 for pdp endian.
                    : HL_UNKNOWN_ENDIAN;    // Else return -1 for wtf endian.
}