Remove AC_FUNC_VPRINTF

Remove AC_FUNC_VPRINTF

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

This macro checks for presence of the vprint function otherwise checks
for presence of the _doprnt function. This check was relevant on very
old systems and today can be omitted since it should be well supported
by now. [2]

Also PHP doesn't use the HAVE_VPRINTF or HAVE_DOPRNT symbols.

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