remove HAVE_DATE_TIME

__DATE__ and __TIME__ are C99 standard predefined macros.
remove HAVE_FLOAT_FUNCS
remove HAVE_RENAME

rename is a C99 standard function.
remove HAVE_SIGCONTEXT
remove HAVE_STDLIB_H
remove HAVE_STRERROR

strerror is a C99 standard function.
remove HAVE_STRFTIME

strftime is a C99 standard function.
remove HAVE_STRING_H
remove HAVE_STRPBRK

strpbrk is a C99 standard function.
remove HAVE_STRTOL

strtol is a C99 standard function.
remove HAVE_SVR4_PTYS
remove HAVE_SYSINFO and HAVE_SYSINFO_MEM_UNIT
remove HAVE_SYS_{POLL_H, RESOURCE_H, SYSCTL_H, TYPES_H}
remove HAVE_USLEEP
remove HAVE_WCHAR_H
remove HAVE_WCTYPE_H
remove HAVE_DL{OPEN, SYM}
remove HAVE_FCHDIR
Replace use of mch_rename with rename.

The macro mch_rename is no longer necessary, because C99 function rename
is always available.