libssh2: fix build with libgcrypt and mbedtls enabled

libssh2: fix build with libgcrypt and mbedtls enabled

The --without-libgcrypt configure parameter is not enough to disable the
libgcrypt back-end. This breaks the build when mbedtls (default backend) is
enabled, and libgcrypt builds before libssh2. Seed ac_cv_libgcrypt to really
disable libgcrypt when it is not the selected crypto back-end.

Fixes:
http://autobuild.buildroot.net/results/b4e/b4e39890e003e9e590a5bf17c9300c653020d88c/
http://autobuild.buildroot.net/results/45a/45a9b3559ceea4e1399794f9612f717f7c57a4a3/
http://autobuild.buildroot.net/results/ae4/ae4afec62ca297df1cd02350601f1d62fc3cd1b7/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>