diff --git a/src/doomtype.h b/src/doomtype.h
index c59e17381..617fd4d47 100644
--- a/src/doomtype.h
+++ b/src/doomtype.h
@@ -100,7 +100,10 @@ typedef long ssize_t;
 	#define strnicmp(x,y,n) strncasecmp(x,y,n)
 #endif
 
-char *strcasestr(const char *in, const char *what);
+char *nongnu_strcasestr(const char *in, const char *what);
+#ifndef _GNU_SOURCE
+#define strcasestr nongnu_strcasestr
+#endif
 #define stristr strcasestr
 
 int startswith (const char *base, const char *tag);
diff --git a/src/strcasestr.c b/src/strcasestr.c
index 2796f11d5..37899a842 100644
--- a/src/strcasestr.c
+++ b/src/strcasestr.c
@@ -61,7 +61,7 @@ swapp (char ***ppap, char ***ppbp, char **cpap, char **cpbp)
 }
 
 char *
-strcasestr (const char *s, const char *q)
+nongnu_strcasestr (const char *s, const char *q)
 {
 	size_t  qn;