Remove useless configure check

The test for "decltype" as a variant of "typeof" apparently never
worked (see also commit 3582b223d4), so remove it.

Discussion: https://www.postgresql.org/message-id/flat/795b1c54-c64a-47f9-8fa3-880dcab59975%40eisentraut.org

config/c-compiler.m4

@@ -142,7 +142,7 @@ fi])# PGAC_C_STATIC_ASSERT
 AC_DEFUN([PGAC_C_TYPEOF],
 [AC_CACHE_CHECK(for typeof, pgac_cv_c_typeof,
 [pgac_cv_c_typeof=no
-for pgac_kw in typeof __typeof__ decltype; do
+for pgac_kw in typeof __typeof__; do
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([],
 [int x = 0;
 $pgac_kw(x) y;

configure

@@ -14344,7 +14344,7 @@ if ${pgac_cv_c_typeof+:} false; then :
   $as_echo_n "(cached) " >&6
 else
   pgac_cv_c_typeof=no
-for pgac_kw in typeof __typeof__ decltype; do
+for pgac_kw in typeof __typeof__; do
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 

meson.build

@@ -2540,7 +2540,7 @@ endif
 
 # Check if the C compiler understands typeof or a variant.  Define
 # HAVE_TYPEOF if so, and define 'typeof' to the actual key word.
-foreach kw : ['typeof', '__typeof__', 'decltype']
+foreach kw : ['typeof', '__typeof__']
   if cc.compiles('''
 int main(void)
 {