diff --git a/CMakeLists.txt b/CMakeLists.txt
index d821cd88c..cb23beb5a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -322,7 +322,7 @@ else()
   endif()
   if(MINGW)
     set(WARNINGS "${WARNINGS} -Wno-error=unused-value -Wno-error=unused-but-set-variable")
-    set(MINGW_FLAG "${MINGW_FLAG} -DWIN32_LEAN_AND_MEAN")
+    set(MINGW_FLAG "${MINGW_FLAG} -DWIN32_LEAN_AND_MEAN -D_POSIX_C_SOURCE")
     set(Boost_THREADAPI win32)
     include_directories(SYSTEM src/platform/mingw)
     # mingw doesn't support LTO (multiple definition errors at link time)