Fix 32-bit MSVC compiler error due to unknown command #warning

[thbeu]

Resolves #2201

[yhirose]

Is it enough to change only this line to #pragma message(...)?

[thbeu]

Well, I only fixed the compiler error and did not touch the warning case on purpose.

[yhirose]

@thbeu how about this? It's simpler and no code duplication.

/*
 * Platform compatibility check
 */

#if defined(_WIN32) && !defined(_WIN64)
#if defined(_MSC_VER)
#pragma message(                                                               \
    "cpp-httplib doesn't support 32-bit Windows. Please use a 64-bit compiler.")
#else
#warning                                                                       \
    "cpp-httplib doesn't support 32-bit Windows. Please use a 64-bit compiler."
#endif
#elif defined(__SIZEOF_POINTER__) && __SIZEOF_POINTER__ < 8
#warning                                                                       \
    "cpp-httplib doesn't support 32-bit platforms. Please use a 64-bit compiler."
#elif defined(__SIZEOF_SIZE_T__) && __SIZEOF_SIZE_T__ < 8
#warning                                                                       \
    "cpp-httplib doesn't support platforms where size_t is less than 64 bits."
#endif