xserver_xorg-server: provide more options for SHA1 library

Until now, the X.org server was forcefully selecting OpenSSL, just to
have a SHA1 implementation. However, in fact, the X.org server is
capable of using a SHA1 implementation from multiple libraries:
OpenSSL, libgcrypt and libsha1 amongst others.

So, this patch changes the X.org server package so that we use the
SHA1 functions from OpenSSL is OpenSSL is already available, or the
SHA1 functions from libgcrypt is libgcrypt is already available, or if
neither OpenSSL nor libgcrypt are enabled in the configuration, we
select the much smaller libsha1, that has been specifically written to
fulfill the X.org server requirements.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>