soapysdr: g++ 15.1.0 causes warning about deprecation of ciso646

[chrisjohgorman]

• #warning " is deprecated in C++17, use to detect implementation-specific macros"
• SoapySDR can build without this file. It appears it was used to allow for and, or and not instead of &&, ||, !.
• tested if this was needed in soapysdr, soapyrtlsdr and soapyhackrf and they all build properly without the header.

[zuckschwerdt]

There is a discussion in #460

[chrisjohgorman]

Thanks @zuckschwerdt. I'm trying to build gnuradio4 and I am building with warnings as errors. I will just turn warnings as errors off. (Unless you want the change.)

[guruofquality]

this was basically for some older versions msvc, removing the includes for iso646 in the types headers isnt a bad idea

maybe a macro with the version could be more robust

#if ((defined(_MSVC_LANG) && _MSVC_LANG >= 201703L) || __cplusplus >= 201703L)
  //C++17 specific stuff here
#else
  #include <ciso646>
#endif

[zuckschwerdt]

And it should be removed at https://github.com/pothosware/SoapySDR/blob/master/include/SoapySDR/Config.h#L51 too -- the C headers and code are not using any replacement operators.