If no botan-config was found we would end up doing:
checking for botan-config... no
/scratch/src/bind10/configure: line 16704: --libs: command not found
/scratch/src/bind10/configure: line 16705: --cflags: command not found

Instead, do not run botan-config if there was no botan-config found
as we will bail out gracefully later on if the botan headers could not
be found.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>