BuildSystem: Avoid obsolete warning with AC_FUNC_SETVBUF_REVERSED on autoconf.

Removed the obsolete macro AC_FUNC_SETVBUF_REVERSED because Asterisk does not
support the platform SVR2 from the year 1987 anymore.

ASTERISK-26046

Change-Id: I28161b037feb2d29ab46ed20e785928460226c22

configure

@@ -15311,13 +15311,6 @@ _ACEOF
 rm -f conftest*
-if ${ac_cv_func_setvbuf_reversed+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  ac_cv_func_setvbuf_reversed=no
-fi
-
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking return type of signal handlers" >&5
 $as_echo_n "checking return type of signal handlers... " >&6; }
 if ${ac_cv_type_signal+:} false; then :

configure.ac

@@ -686,7 +686,6 @@ AC_PROG_GCC_TRADITIONAL
 AC_FUNC_MEMCMP
 AC_FUNC_MMAP
 AC_FUNC_SELECT_ARGTYPES
-AC_FUNC_SETVBUF_REVERSED
 AC_TYPE_SIGNAL
 AC_FUNC_STAT
 AC_FUNC_STRCOLL