If long double math functions do not work, drop uselongdouble.

This moves a earlier version of the test from the freebsd hints
to Configure, thus stopping also any other platforms that have
lacking longdouble implementations.

The test is not comprehensive (not all long double interfaces are tested),
but it covers some of the most common ones.

The earlier test was actually wrong, so no FreeBSD could ever pass.
Sorry, FreeBSD.
Revert "sisyphus thinks the test value is simply wrong here."

This reverts commit 5399a05dbb94d8e0c2a0ea2b588ff80ad23f2093.

(sisyphus thinks the bug is actually in ppc64 linux exp())
Special case exp(1) for ppc64-linux.