auxiliary/os: Don't implement os_get_option() on embedded builds.

Let it be defined externally instead, allowing setting mechanisms other
than environment variables.

Reviewed-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Matthew McClure <mcclurem@vmware.com>

src/gallium/auxiliary/os/os_misc.c

@@ -96,11 +96,13 @@ os_log_message(const char *message)
 }
 
 
+#if !defined(PIPE_SUBSYSTEM_EMBEDDED)
 const char *
 os_get_option(const char *name)
 {
    return getenv(name);
 }
+#endif /* !PIPE_SUBSYSTEM_EMBEDDED */
 
 
 /**