* numbers.c (s_scm_make_polar): Use sincos, when available.
        (scm_magnitude): Use hypot.
        * configure.in (AC_CHECK_FUNCS): Add sincos.
        * tests/numbers.test (make-polar, magnitude): Add tests.

glibc sincos is supposedly more efficient than separate sin and cos
calls.

hypot should give greater accuracy than a sqrt expression.  It dates
back at least to v7 unix and so shouldn't need a configure test.