bootstrap: Fix support for Intel compiler with modern GNU system compiler

On systems with older GNU system compilers, the Intel C++ compiler does
not define `__cplusplus` to any version newer than C++11.  This
prevented `bootstrap` from detecting that a given C++ standard flag has
enabled C++17 mode in the compiler.  In commit 033a4b12a5 (bootstrap:
Extend C++17 check for our cast functions, 2019-12-14,
v3.17.0-rc1~291^2) we added a preprocessor condition to attempt to
detect C++17 mode in the Intel compiler on such systems by looking
for `__cpp_if_constexpr`.  However, on systems with a modern GNU
system compiler, that definition is available even in C++11 mode.

Switch to using `__cpp_deduction_guides` to detect C++17 mode for the
Intel C++ compiler.  That seems to be defined exclusively in C++17 mode
regardless of the version of the system compiler.

Fixes: #21013

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