Bug 1883858 - Remove configure checks that no longer make sense r=glandium

AC_C_CONST is officially useless, see https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C-Compiler.html
AC_TYPE_SIZE_T is probably as useless, size_t is a standard type in C99 (from stddef.h)
AC_EXEEXT sets EXEEXT which is never used (and the internal ac_cv_exeext which is not used either)
MISSING_X is never referenced

Differential Revision: https://phabricator.services.mozilla.com/D203732

js/src/old-configure.in

@@ -30,10 +30,6 @@ dnl Set the minimum version of toolkit libs used by mozilla
 dnl ========================================================
 W32API_VERSION=3.14
 
-dnl Set various checks
-dnl ========================================================
-MISSING_X=
-
 dnl Initialize the Pthread test variables early so they can be
 dnl  overridden by each platform.
 dnl ========================================================
@@ -58,9 +54,6 @@ dnl ========================================================
 
 if test "$COMPILE_ENVIRONMENT"; then
 
-# Run some logic to figure out exe extensions (mostly for mingw's sake)
-AC_EXEEXT
-
 # Note:
 #   In Mozilla, we use the names $target, $host and $build incorrectly, but are
 #   too far gone to back out now. See Bug 475488:
@@ -440,12 +433,10 @@ fi
 if test -z "$SKIP_COMPILER_CHECKS"; then
 dnl Checks for typedefs, structures, and compiler characteristics.
 dnl ========================================================
-AC_C_CONST
 AC_TYPE_MODE_T
 AC_TYPE_OFF_T
 AC_TYPE_PID_T
-AC_TYPE_SIZE_T
-AC_LANG_CPLUSPLUS
+
 AC_LANG_C
 AC_MSG_CHECKING(for ssize_t)
 AC_CACHE_VAL(ac_cv_type_ssize_t,

old-configure.in

@@ -30,10 +30,6 @@ dnl Set the minimum version of toolkit libs used by mozilla
 dnl ========================================================
 W32API_VERSION=3.14
 
-dnl Set various checks
-dnl ========================================================
-MISSING_X=
-
 dnl Initialize the Pthread test variables early so they can be
 dnl  overridden by each platform.
 dnl ========================================================
@@ -53,9 +49,6 @@ dnl ========================================================
 
 if test "$COMPILE_ENVIRONMENT"; then
 
-# Run some logic to figure out exe extensions (mostly for mingw's sake)
-AC_EXEEXT
-
 if test "$target" != "$host"; then
     MOZ_CROSS_COMPILER
 else
@@ -473,13 +466,9 @@ fi # COMPILE_ENVIRONMENT
 if test -z "$SKIP_COMPILER_CHECKS"; then
 dnl Checks for typedefs, structures, and compiler characteristics.
 dnl ========================================================
-AC_C_CONST
 AC_TYPE_MODE_T
 AC_TYPE_OFF_T
 AC_TYPE_PID_T
-AC_TYPE_SIZE_T
-AC_LANG_CPLUSPLUS
-AC_LANG_C
 
 AC_LANG_CPLUSPLUS