diff --git a/tools/mgba-rom-test-hydra/Makefile b/tools/mgba-rom-test-hydra/Makefile
index f93f991d9b38..ef0523adc9d1 100644
--- a/tools/mgba-rom-test-hydra/Makefile
+++ b/tools/mgba-rom-test-hydra/Makefile
@@ -12,7 +12,7 @@ all: mgba-rom-test-hydra$(EXE)
 	@:
 
 mgba-rom-test-hydra$(EXE): $(SRCS)
-	$(CC) $(SRCS) -o $@ -lm $(LDFLAGS)
+	$(CC) $(SRCS) -Werror=implicit-function-declaration -o $@ -lm $(LDFLAGS)
 
 clean:
 	$(RM) mgba-rom-test-hydra$(EXE)
diff --git a/tools/mgba-rom-test-hydra/main.c b/tools/mgba-rom-test-hydra/main.c
index 841b7328e5fa..d4c0f4e080cc 100644
--- a/tools/mgba-rom-test-hydra/main.c
+++ b/tools/mgba-rom-test-hydra/main.c
@@ -106,6 +106,28 @@ static const struct Symbol *lookup_address(uint32_t address)
     return NULL;
 }
 
+#ifndef _GNU_SOURCE
+// Very naive implementation of 'memmem' for systems which don't make it
+// available by default.
+void *memmem(const void *haystack, size_t haystacklen, const void *needle, size_t needlelen)
+{
+    const char *haystack_ = haystack;
+    const char *needle_ = needle;
+    for (size_t i = 0; i < haystacklen - needlelen; i++)
+    {
+        size_t j;
+        for (j = 0; j < needlelen; j++)
+        {
+            if (haystack_[i+j] != needle_[j])
+                break;
+        }
+        if (j == needlelen)
+            return (void *)&haystack_[i];
+    }
+    return NULL;
+}
+#endif
+
 // Similar to 'fwrite(buffer, 1, size, f)' except that anything which
 // looks like the output of '%p' (i.e. '<0x\d{7}>') is translated into
 // the name of a symbol (if it represents one).