libgcrypt: do not test for /dev/* files if getentropy is available

[Steffen Nurpmeso]

Hello.

Ha, i just realized that i am indeed subscribed to gnupg-devel, so
i copy over the patch i had posted to gnupg-users@ in thread
  libgcrypt: random source via library on Linux?
but all united in one.

Ciao,

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)
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