enca: fix S!=B failure

Point tools to the right paths:
* ${S}/configure.ac in do_configure_prepare()
* ${B}/Makefile in do_configure_append()
* cd ${B} in do_compile()

Signed-off-by: Tim Orling <TicoTimo@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>