diff --git a/include/bsd/stdlib.h b/include/bsd/stdlib.h
index 8d33d1f..a5b063c 100644
--- a/include/bsd/stdlib.h
+++ b/include/bsd/stdlib.h
@@ -71,7 +71,8 @@ int sradixsort(const unsigned char **base, int nmemb,
                const unsigned char *table, unsigned endbyte);
 
 void *reallocf(void *ptr, size_t size);
-#if defined(_GNU_SOURCE) && defined(__GLIBC__) && !__GLIBC_PREREQ(2, 26)
+#if !defined(__GLIBC__) || \
+    (defined(__GLIBC__) && (!__GLIBC_PREREQ(2, 26) || !defined(_GNU_SOURCE)))
 void *reallocarray(void *ptr, size_t nmemb, size_t size);
 #endif
 
diff --git a/include/bsd/string.h b/include/bsd/string.h
index 29097f6..f987fee 100644
--- a/include/bsd/string.h
+++ b/include/bsd/string.h
@@ -46,7 +46,8 @@ size_t strlcat(char *dst, const char *src, size_t siz);
 char *strnstr(const char *str, const char *find, size_t str_len);
 void strmode(mode_t mode, char *str);
 
-#if defined(_GNU_SOURCE) && defined(__GLIBC__) && !__GLIBC_PREREQ(2, 25)
+#if !defined(__GLIBC__) || \
+    (defined(__GLIBC__) && (!__GLIBC_PREREQ(2, 25) || !defined(_GNU_SOURCE)))
 void explicit_bzero(void *buf, size_t len);
 #endif
 __END_DECLS
diff --git a/include/bsd/sys/cdefs.h b/include/bsd/sys/cdefs.h
index b4c8f30..d1cc419 100644
--- a/include/bsd/sys/cdefs.h
+++ b/include/bsd/sys/cdefs.h
@@ -58,6 +58,14 @@
 #endif
 #endif
 
+/*
+ * On non-glibc based systems, we cannot unconditionally use the
+ * __GLIBC_PREREQ macro as it gets expanded before evaluation.
+ */
+#ifndef __GLIBC_PREREQ
+#define __GLIBC_PREREQ(maj, min) 0
+#endif
+
 /*
  * Some kFreeBSD headers expect those macros to be set for sanity checks.
  */