diff --git a/src/core/features.h b/src/core/features.h
index e74f09d2..d0b52139 100644
--- a/src/core/features.h
+++ b/src/core/features.h
@@ -25,6 +25,8 @@
 #ifndef JANET_FEATURES_H_defined
 #define JANET_FEATURES_H_defined
 
+#ifndef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #endif
diff --git a/tools/amalg.janet b/tools/amalg.janet
index b1806ab3..cca3dde6 100644
--- a/tools/amalg.janet
+++ b/tools/amalg.janet
@@ -5,6 +5,7 @@
 (print "/* Generated from janet version " janet/version "-" janet/build " */")
 (print "#define JANET_BUILD \"" janet/build "\"")
 (print ```#define JANET_AMALG```)
+(print ```#define _POSIX_C_SOURCE 200112L```)
 (print ```#include "janet.h"```)
 
 # Body