Actually set the execinfo flag on non-glibc systems

(cherry picked from commit a3b5353892)

platform/x11/detect.py

@@ -404,6 +404,7 @@ def configure(env):
         # The default crash handler depends on glibc, so if the host uses
         # a different libc (BSD libc, musl), fall back to libexecinfo.
         print("Note: Using `execinfo=yes` for the crash handler as required on platforms where glibc is missing.")
+        env["execinfo"] = True
 
     if env["execinfo"]:
         env.Append(LIBS=["execinfo"])