[Bug preprocessor/96952] __builtin_thread_pointer support cannot be probed

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96952

--- Comment #5 from Rich Felker <bugdal at aerifal dot cx> ---
The whole point of __has_builtin is to let you avoid the configure-time checks
on compilers that support __has_builtin. If __has_builtin doesn't actually
work, it's pointless that it even exists and indeed everyone should just
pretend it doesn't exist and keep using configure-time checks for everything.