diff --git a/Makefile.am b/Makefile.am
index af437a6..0a7b929 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1 +1,2 @@
+ACLOCAL_AMFLAGS = -I m4
 SUBDIRS = src
diff --git a/configure.ac b/configure.ac
index 913aa44..fd0dbbe 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,3 +1,6 @@
+# This file uses AC_CONFIG_MACRO_DIR, so we need Autoconf 2.58 or better.
+AC_PREREQ([2.58])
+
 AC_INIT([WrapSix], [0.1.99.0], [xhire@mujmalysvet.cz])
 AM_INIT_AUTOMAKE([-Wall -Werror])
 
@@ -10,6 +13,7 @@ AC_CONFIG_FILES([
 	Makefile
 	src/Makefile
 ])
+AC_CONFIG_MACRO_DIR([m4])
 
 ###
 # Configuration options
@@ -21,10 +25,10 @@ AC_ARG_ENABLE([debug],
 	      [debug=no])
 
 if test "x$debug" = "xyes"; then
-	AM_CFLAGS="${AM_CFLAGS} -g -ggdb -O0 -pipe -pedantic -Wshadow -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -Wold-style-definition -Wdeclaration-after-statement -Wmissing-declarations -Wmissing-prototypes -Wredundant-decls -Wshadow -Wpointer-arith -Wcast-align -Wwrite-strings -Winline -Wformat-nonliteral -Wformat-security -Wswitch-enum -Winit-self -Wmissing-include-dirs -Wundef -Waggregate-return -Wnested-externs -Wunsafe-loop-optimizations"
+	AS_COMPILER_FLAGS(AM_CFLAGS, "-g -ggdb -O0 -pipe -pedantic -Wshadow -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -Wold-style-definition -Wdeclaration-after-statement -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wredundant-decls -Wmissing-noreturn -Wshadow -Wpointer-arith -Wcast-align -Wwrite-strings -Winline -Wformat-nonliteral -Wformat-security -Wswitch-enum -Winit-self -Wmissing-include-dirs -Wundef -Waggregate-return -Wnested-externs -Wunsafe-loop-optimizations -Winvalid-pch")
 	AC_DEFINE([DEBUG])
 else
-        AM_CFLAGS="${AM_CFLAGS} -O2"
+        AS_COMPILER_FLAGS(AM_CFLAGS, "-O2")
 fi
 
 ###
diff --git a/m4/as-compiler-flag.m4 b/m4/as-compiler-flag.m4
new file mode 100644
index 0000000..1c74267
--- /dev/null
+++ b/m4/as-compiler-flag.m4
@@ -0,0 +1,61 @@
+dnl as-compiler-flag.m4 0.1.0
+
+dnl autostars m4 macro for detection of compiler flags
+
+dnl David Schleef <ds@schleef.org>
+
+dnl $Id: as-compiler-flag.m4,v 1.1 2005/12/15 23:35:19 ds Exp $
+
+dnl AS_COMPILER_FLAG(CFLAGS, ACTION-IF-ACCEPTED, [ACTION-IF-NOT-ACCEPTED])
+dnl Tries to compile with the given CFLAGS.
+dnl Runs ACTION-IF-ACCEPTED if the compiler can compile with the flags,
+dnl and ACTION-IF-NOT-ACCEPTED otherwise.
+
+AC_DEFUN([AS_COMPILER_FLAG],
+[
+  AC_MSG_CHECKING([to see if compiler understands $1])
+
+  save_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $1"
+
+  AC_TRY_COMPILE([ ], [], [flag_ok=yes], [flag_ok=no])
+  CFLAGS="$save_CFLAGS"
+
+  if test "X$flag_ok" = Xyes ; then
+    m4_ifvaln([$2],[$2])
+    true
+  else
+    m4_ifvaln([$3],[$3])
+    true
+  fi
+  AC_MSG_RESULT([$flag_ok])
+])
+
+dnl AS_COMPILER_FLAGS(VAR, FLAGS)
+dnl Tries to compile with the given CFLAGS.
+
+AC_DEFUN([AS_COMPILER_FLAGS],
+[
+  list=$2
+  flags_supported=""
+  flags_unsupported=""
+  AC_MSG_CHECKING([for supported compiler flags])
+  for each in $list
+  do
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="$CFLAGS $each"
+    AC_TRY_COMPILE([ ], [], [flag_ok=yes], [flag_ok=no])
+    CFLAGS="$save_CFLAGS"
+
+    if test "X$flag_ok" = Xyes ; then
+      flags_supported="$flags_supported $each"
+    else
+      flags_unsupported="$flags_unsupported $each"
+    fi
+  done
+  AC_MSG_RESULT([$flags_supported])
+  if test "X$flags_unsupported" != X ; then
+    AC_MSG_WARN([unsupported compiler flags: $flags_unsupported])
+  fi
+  $1="$$1 $flags_supported"
+])