Remove AC_STRUCT_TM macro

Remove AC_STRUCT_TM macro

Autoconf 2.59d (released in 2006) [1] started promoting several macros
as not relevant for newer systems anymore, including the `AC_STRUCT_TM`.

This macro checks if `struct tm` is defined in either `<sys/time.h>` or
`<time.h>` and defines the `TM_IN_SYS_TIME` symbol accordingly. This
check was relevant in times before the C89 for some embedded systems,
microcontrollers or very old systems. For newer systems it can be
avoided and the `<time.h>` should be included instead since current
systems should be well supported by now. [2]

Since PHP requires at least C89, this patch removes the obsolescent call
and time.h checks.

Refs:

- [1]: http://git.savannah.gnu.org/cgit/autoconf.git/tree/NEWS
- [2]: https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Particular-Structures.html