diff --git a/configure.ac b/configure.ac
index 920612f..06d5dfa 100644
--- a/configure.ac
+++ b/configure.ac
@@ -147,6 +147,7 @@ AC_CHECK_DECLS([TIOCSRS485], [], [], [[#include <sys/ioctl.h>]])
 # Check for RTS flags
 AC_CHECK_DECLS([TIOCM_RTS], [], [], [[#include <sys/ioctl.h>]])
 
+test -n "$WARNING_CFLAGS" || \
 WARNING_CFLAGS="-Wall \
 -Wmissing-declarations -Wmissing-prototypes \
 -Wnested-externs -Wpointer-arith \
@@ -178,6 +179,7 @@ AC_ARG_ENABLE([debug],
   [enable_debug=$enableval],
   [enable_debug=no])
 
+test -n "$CFLAGS" || \
 AS_IF([test "x$enable_debug" = "xyes"], [
   CFLAGS="-g -O0"
   CXXFLAGS="-g -O0"