diff --git a/configure.ac b/configure.ac
index 4cbebdab..752930f3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4,7 +4,7 @@ AM_INIT_AUTOMAKE([foreign])
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4])
 
-CXXFLAGS=-O3
+CXXFLAGS="${CXXFLAGS} -O3"
 
 AC_PROG_CC
 AC_PROG_CXX