sqrtl: Bypass broken implementation in OpenBSD 5.1/SPARC.

sqrtl: Bypass broken implementation in OpenBSD 5.1/SPARC.

* lib/math.in.h (sqrtl): Replace it if REPLACE_SQRTL is 1.
* m4/sqrtl.m4 (gl_FUNC_SQRTL_WORKS): New macro.
(gl_FUNC_SQRTL): Invoke it. Set REPLACE_SQRTL to 1 if sqrtl() produces
too big rounding errors.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_SQRTL.
* modules/math (Makefile.am): Substitute REPLACE_SQRTL.
* modules/sqrtl (configure.ac): Consider REPLACE_SQRTL.
(Depends-on): Update conditions.
* tests/test-sqrtl.c (my_ldexpl): New function.
(main): Add test of a particular value.
* doc/posix-functions/sqrtl.texi: Mention the OpenBSD 5.1/SPARC bug.