gnutls: fix libgcrypt handling with external toolchains

The explicit gnutls prefix in combination with the libc.so linker script
and non-default sysroot seems to confuse external toolchains, causing
configure to error out. Workaround it by not using an explicit libgcrypt
prefix as it is installed in the sysroot anyway.

Fixes http://autobuild.buildroot.net/results/3b3901bea904bf32bccbd3e9a4274d604776cb98



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>