fix out of tree build

fix out of tree build

In addition to use ${srcdir} and deliver ${NOTMUCH_SRCDIR} where needed,
source from ruby bindings had to be copied to the out-of-tree target
directory -- if the source files in source directory were referenced
in build and there were also built object files there, those could have
been considered as target files (and then not found when attempting
to create bindings/ruby/notmuch.so).