--- a/build/make/gen_msvs_vcxproj.sh
+++ b/build/make/gen_msvs_vcxproj.sh
@@ -209,7 +209,7 @@
         ;;
         -l*) libs="${libs}${libs:+ }${opt##-l}.lib"
         ;;
-        -*) die_unknown $opt
+        -*) : # Ignore unknown flags (e.g. -O3 leaked from GCC CFLAGS)
         ;;
         *)
             # The paths in file_list are fixed outside of the loop.