float, math: Fix 'int' to 'long double' conversion on Linux/SPARC64.

float, math: Fix 'int' to 'long double' conversion on Linux/SPARC64.

* m4/float_h.m4 (gl_FLOAT_H): Test conversion from 'int' to
'long double'. Set REPLACE_ITOLD.
* lib/float.in.h (_Qp_itoq, _gl_float_fix_itold): New declarations.
* lib/math.in.h (_Qp_itoq, _gl_math_fix_itold): New declarations.
* lib/itold.c: New file.
* modules/float (Files): Add lib/itold.c.
(configure.ac): When REPLACE_ITOLD is 1, arrange to compile itold.c.
(Makefile.am): Substitute REPLACE_ITOLD.
* modules/math (Depends-on): Add float.
(Makefile.am): Substitute REPLACE_ITOLD.
* doc/posix-headers/float.texi: Mention problem on Linux/SPARC64.
* doc/posix-headers/math.texi: Likewise.
* doc/posix-functions/logl.texi: Likewise.