Move version files out of extern folder

The folder extern is reserved for external contributions, not common
files.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/version.c

@@ -0,0 +1,26 @@
+/*
+ * This file is part of RGBDS.
+ *
+ * Copyright (c) 2017-2018, Antonio Nino Diaz and RGBDS contributors.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "version.h"
+
+const char *get_package_version_string(void)
+{
+	static char s[50];
+
+	/* The following conditional should be simplified by the compiler. */
+	if (strlen(BUILD_VERSION_STRING) == 0) {
+		snprintf(s, sizeof(s), "v%d.%d.%d", PACKAGE_VERSION_MAJOR,
+			 PACKAGE_VERSION_MINOR, PACKAGE_VERSION_PATCH);
+		return s;
+	} else {
+		return BUILD_VERSION_STRING;
+	}
+}