build: libebook: Fix race condition in gen-western-table.py

build: libebook: Fix race condition in gen-western-table.py

In a parallel build, the normal compiler process will race with the
process writing to the table header.  Depending on timing, it may or
may not work.

Fix this in two ways:

* Write the file atomically, so #include either gets the whole thing,
  or nothing.
* Add it to BUILT_SOURCES so automake knows to build it first.