strcasecmp: Work around Solaris, Cygwin bug. * lib/strings.in.h (strcasecmp): Consider REPLACE_STRCASECMP. Use the usual idioms. * m4/strings_h.m4 (gl_STRINGS_H_DEFAULTS): Initialize REPLACE_STRCASECMP. * m4/strcasecmp.m4 (gl_STRCASECMP_WORKS): New macro. (gl_FUNC_STRCASECMP): Invoke it. Set REPLACE_STRCASECMP. * modules/strcasecmp (configure.ac): Consider REPLACE_STRCASECMP. * modules/strings-h (Makefile.am): Substitute REPLACE_STRCASECMP. * doc/posix-functions/strcasecmp.texi: Mention the Solaris, Cygwin bug.