Let autoconf detect -g and -O2, do not reinvent the wheel

Save CFLAGS and CXXFLAGS after the compiler is detected, rather than
before. This is when autoconf sets -g and -O2 automatically by default.

configure.ac

@@ -49,11 +49,6 @@ AC_SUBST(vlcdatadir)
 vlclibdir="\${libdir}/\${PKGDIR}"
 AC_SUBST(vlclibdir)
 
-dnl
-dnl  Save *FLAGS
-dnl
-VLC_SAVE_FLAGS
-
 dnl
 dnl Check for tools
 dnl
@@ -87,6 +82,11 @@ AC_C_CONST
 AC_C_INLINE
 AC_C_RESTRICT
 
+dnl
+dnl  Save *FLAGS
+dnl
+VLC_SAVE_FLAGS
+
 dnl Extend the --help string at the current spot.
 AC_DEFUN([EXTEND_HELP_STRING], [m4_divert_once([HELP_ENABLE], [$1])])
 
@@ -921,16 +921,12 @@ dnl  Debugging mode
 dnl
 AC_ARG_ENABLE(debug,
   [AS_HELP_STRING([--enable-debug],
-    [build with run-time assertions and debug symbols (default disabled)])],,
+    [build with run-time assertions (default disabled)])],,
   [enable_debug="no"])
 AH_TEMPLATE(NDEBUG,
             [Define to 1 if debug code should NOT be compiled])
 AS_IF([test "${enable_debug}" != "no"], [
   AC_CHECK_HEADERS([valgrind/valgrind.h])
-  CFLAGS="${CFLAGS} -g"
-  CXXFLAGS="${CXXFLAGS} -g"
-  OBJCFLAGS="${OBJCFLAGS} -g"
-  LDFLAGS="${LDFLAGS} -g"
 ], [
   AC_DEFINE(NDEBUG)
 ])
@@ -1035,33 +1031,6 @@ AS_IF([test "${enable_optimizations}" != "no"], [
     ])
   ])
 
-  dnl -O2 and -O in both production and debug builds
-  AS_IF([test "x$C_O" = "x"], [
-    AC_CACHE_CHECK([if \$CC accepts -O2], [ac_cv_c_o2], [
-      CFLAGS="${CFLAGS_save} -O2"
-      AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [
-        ac_cv_c_o2=yes
-      ], [
-        ac_cv_c_o2=no
-      ])
-    ])
-    AS_IF([test "${ac_cv_c_o2}" != "no"], [
-      C_O="-O2"
-    ], [
-      AC_CACHE_CHECK([if \$CC accepts -O], [ac_cv_c_o], [
-        CFLAGS="${CFLAGS_save} -O"
-        AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [
-          ac_cv_c_o=yes
-        ], [
-          ac_cv_c_o=no
-        ])
-      ])
-      AS_IF([test "${ac_cv_c_o}" != "no"], [
-        C_O="-O"
-      ])
-    ])
-  ])
-
   VLC_RESTORE_FLAGS
   CFLAGS="${CFLAGS} ${C_O}"
   CXXFLAGS="${CXXFLAGS} ${C_O}"