Add out-of-the-box support for compiling with Intel Compiler v19.0

[sharpe5]

PR for Issue #1273.

Out of the box, the Intel Compiler v19.0 does not understand the [[deprecated]] attribute and generates a compiler error:

1>.\DemoFmt\github\fmt\include\fmt\core.h(478): error : attribute does not apply to any entity
1>  using wparse_context FMT_DEPRECATED = basic_parse_context<wchar_t>;
1>                       ^

However, it does accept the [[gnu:deprecated]] attribute.

I have updated the following StackOverflow question to document this:
https://stackoverflow.com/questions/295120/c-mark-as-deprecated/57680802#57680802

Tested with the following language levels:

• C++17 Support (/Qstd=c++17)
• C++14 Support (/Qstd=c++14)
• C++11 Support (/Qstd=c++11)
• C11 Support (/Qstd=c11)
• C99 Support (/Qstd=c99)

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[sharpe5]

To test, install the Intel Compiler v19.0, the use the following in a header-only mode:

#include "pch.h"

#define FMT_HEADER_ONLY 
#include <fmt/core.h> 
#include <fmt/format.h>
#include <iostream>

int main()
{
    std::cout << "Hello World!\n";
    fmt::print("Hello, {}!", "world");
}

[sharpe5]

To add specific support for Intel Compiler v19.00, this works:

#if (FMT_HAS_CPP_ATTRIBUTE(deprecated)) && __INTEL_COMPILER >= 1900
#    define FMT_DEPRECATED [[gnu::deprecated]]
#endif

I'm not sure how far back [[gnu::deprecated]] is supported by the Intel Compiler.

According to this page, the Intel Compiler does support the [deprecated] attribute, so I think this could almost be considered to be a bug in the Intel Compiler (considering that it is designed to be compatible with MSVC):
https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-std-qstd

[sharpe5]

Closing this request as further testing shows it is not entirely correct.

The Intel Compiler generally accepts [[deprecated]], but it does not accept [[deprecated]] on certain language elements, such as this:

using wparse_context [[deprecated]] = basic_parse_context<wchar_t>;

The use of [[gnu::deprecated]] appears to be misleading, as it also accepts this:

using wparse_context [[gnu::deprecatedZZZZZ]] = basic_parse_context<wchar_t>;