diff --git a/configure.ac b/configure.ac
index 2b6a87e..b8a985f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -12,6 +12,8 @@
 AC_DEFINE([INCOMPLETE_TYPES_BROKEN], 1, [Define if you want use complete types])
 
 # Checks for programs.
+SAVED_CFLAGS=${CFLAGS}
+SAVED_CXXFLAGS=${CXXFLAGS}
 AC_PROG_CPP
 AC_PROG_CC
 AC_PROG_CXX
@@ -19,6 +21,15 @@
 AC_PROG_LIBTOOL
 AC_PROG_LN_S
 
+if test "x${GCC}" = "xyes"; then
+  if test "x${SAVED_CFLAGS}" = "x"; then
+    CFLAGS=-O3
+  fi
+  if test "x${SAVED_CXXFLAGS}" = "x"; then
+    CXXFLAGS=-O3
+  fi
+fi
+
 # Checks for libraries.
 
 # Checks for header files.