added support for -O4 when compiling with --disable-debug

configure.ac

@@ -1017,13 +1017,20 @@ else
     fi
 fi
 
-AC_CACHE_CHECK([if \$CC accepts -O3],
+AC_CACHE_CHECK([if \$CC accepts -O4],
+    [ac_cv_c_o4],
+    [CFLAGS="${CFLAGS_save} -O4"
+     AC_TRY_COMPILE([],,ac_cv_c_o4=yes, ac_cv_c_o4=no)])
+if test "${ac_cv_c_o4}" != "no" -a "x${enable_debug}" = "xno"; then
+    CFLAGS_OPTIM_SPEED="${CFLAGS_OPTIM_SPEED} -O4"
+else
+    AC_CACHE_CHECK([if \$CC accepts -O3],
         [ac_cv_c_o3],
         [CFLAGS="${CFLAGS_save} -O3"
          AC_TRY_COMPILE([],,ac_cv_c_o3=yes, ac_cv_c_o3=no)])
-if test "${ac_cv_c_o3}" != "no" -a "x${enable_debug}" = "xno"; then
+    if test "${ac_cv_c_o3}" != "no" -a "x${enable_debug}" = "xno"; then
         CFLAGS_OPTIM_SPEED="${CFLAGS_OPTIM_SPEED} -O3"
-else
+    else
         AC_CACHE_CHECK([if \$CC accepts -O2],
             [ac_cv_c_o2],
             [CFLAGS="${CFLAGS_save} -O2"
@@ -1039,6 +1046,7 @@ else
                 CFLAGS_OPTIM_SPEED="${CFLAGS_OPTIM_SPEED} -O"
             fi
         fi
+    fi
 fi
 
 AC_CACHE_CHECK([if \$CC accepts -O0],