Regression: lib doesn't build with GCC 10 with c++20 anymore

[Ka0o0]

Hi,
since #67 the build breaks with GCC 10. This is because the language feature c++20 is still experimental and the variable __cplusplus expands to 201709L as per documentation of the compiler: https://gcc.gnu.org/onlinedocs/gcc-10.4.0/cpp/Standard-Predefined-Macros.html#Standard-Predefined-Macros. This is a regression, because previously the library would build with the native span implementation after being installed with CMake and make install. GCC 10 is still shipped with Debian bullseye so I think it's still worth fixing for this compiler version.