Remove AC_C_CONST

Remove AC_C_CONST

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

The `const` keyword is used in C since C89. On old systems some compilers
lacked the `const` and this macro defined it to be empty. This check was
relevant on systems with compilers before C89 and on current systems it
can be omitted. [2]

PHP also requires at least C89 so `const` is always available.

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