Herbie should warn if MPFR is single-threaded

[pavpanchekha]

MPFR can be compiled in single-threaded mode, in which case all MPFR function synchronize. The relevant code (which requires ffi/unsafe and math/private/bigfloat/mpfr) is:

((get-ffi-obj 'mpfr_buildopt_tls_p mpfr-lib (_fun -> _bool)))

This returns #t if MPFR is multi-threaded and #f if it's single-threaded. Perhaps Herbie should warn if you ask for threads but your MPFR is single-threaded.