Fixes for the build system

1. Fix make distcheck and test it with the Buildbot.
2. Restore ./config/default (still needs to be updated for the new software environment on Breeze).
3. Enable the maintainer mode by default. Let's see whether we get any problems with this (see also here).

.ci/bb/levante-dkrz/test.all.gcc

@@ -45,9 +45,9 @@ LDFLAGS="${LDFLAGS}" \
 MPI_LAUNCH="${MPI_LAUNCH}" \
 PKG_CONFIG_PATH="${YAXT_ROOT}/lib/pkgconfig:${PPM_ROOT}/lib/pkgconfig"
 
-make -j8
+make
 
-make -j8 check || { cat tests/test-suite.log; exit 1; }
+make check || { cat tests/test-suite.log; exit 1; }
 
 check_all_tests_passed tests/test-suite.log