Merge #16435: autoconf: Sane `--enable-debug` defaults.

d6ac25bdd9 autoconf: Sane --enable-debug defaults. (Carl Dong)

Pull request description:

  ```
  Don't optimize even if variables adhere to as-if rule. This is a
  somewhat sane default for debugging.
  ```

  -----

  Fixes: #14830

  This is more of a "do something dumb and have people correct you" kind of PR. The end goal is to have a configure flag that will allow for debugging without annoying "optimized out" messages, for developer experiences' sake. This is the minimal diff, but people have suggested `--enable-debug-slow` in the past.

ACKs for top commit:
  jonasschnelli:
    Tested ACK d6ac25bdd9

Tree-SHA512: 7a5576ad1d33850aff1445ccb71b133f654b455da2d1daed2ed1b82ea773965790a62895aeeab74b23a25513ab96dddb670f9dbc593dd0b8c030694206a99ccf

configure.ac

@@ -239,7 +239,7 @@ AM_CONDITIONAL(ENABLE_MAN, test "$enable_man" != no)
 # Enable debug
 AC_ARG_ENABLE([debug],
     [AS_HELP_STRING([--enable-debug],
-                    [use debug compiler flags and macros (default is no)])],
+                    [use compiler flags and macros suited for debugging (default is no)])],
     [enable_debug=$enableval],
     [enable_debug=no])
 
@@ -271,12 +271,9 @@ if test "x$enable_debug" = xyes; then
   if test "x$CXXFLAGS_overridden" = xno; then
 	CXXFLAGS=""
   fi
-  # Prefer -Og, fall back to -O0 if that is unavailable.
-  AX_CHECK_COMPILE_FLAG(
-    [-Og],
-    [[DEBUG_CXXFLAGS="$DEBUG_CXXFLAGS -Og"]],
-    [AX_CHECK_COMPILE_FLAG([-O0],[[DEBUG_CXXFLAGS="$DEBUG_CXXFLAGS -O0"]],,[[$CXXFLAG_WERROR]])],
-    [[$CXXFLAG_WERROR]])
+
+  # Disable all optimizations
+  AX_CHECK_COMPILE_FLAG([-O0], [[DEBUG_CXXFLAGS="$DEBUG_CXXFLAGS -O0"]],,[[$CXXFLAG_WERROR]])
 
   # Prefer -g3, fall back to -g if that is unavailable.
   AX_CHECK_COMPILE_FLAG(