--- a/configure.ac	2019-05-19 18:40:05.000000000 +0200
+++ b/configure.ac	2019-09-21 17:34:53.499993726 +0200
@@ -31,8 +31,8 @@
    CXXFLAGS="-Wall -O0 -g -D_DEBUG"
    AC_MSG_RESULT([yes])
 else
-   CFLAGS="-O3"
-   CXXFLAGS="-O3"
+   CFLAGS?="-O3"
+   CXXFLAGS?="-O3"
    AC_MSG_RESULT([no])
 fi