diff --git a/configure.ac b/configure.ac
index 824b778..2210296 100644
--- a/configure.ac
+++ b/configure.ac
@@ -31,7 +31,7 @@
 )
 AS_IF([test "x$enable_boot_flag_safe_mode_support" != "xno"],
     AC_MSG_NOTICE([Enabling safe mode option in boot flags])
-    [cpp_flags="-DENABLE_BOOT_FLAG_SAFE_MODE_SUPPORT"]
+    [cpp_flags="$cpp_flags -DENABLE_BOOT_FLAG_SAFE_MODE_SUPPORT"]
     AC_SUBST([CPPFLAGS], [$cpp_flags]),
     AC_MSG_WARN([Disabling safe mode option in boot flags])
 )
@@ -42,7 +42,7 @@
 )
 AS_IF([test "x$enable_i2c_whitelist_check" != "xno"],
     AC_MSG_NOTICE([Enabling I2C master write read command white list check])
-    [cpp_flags="-DENABLE_I2C_WHITELIST_CHECK"]
+    [cpp_flags="$cpp_flags -DENABLE_I2C_WHITELIST_CHECK"]
     AC_SUBST([CPPFLAGS], [$cpp_flags]),
     AC_MSG_WARN([Disabling I2C master write read command white list check])
 )