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.icon-pio.intel-classic

@@ -12,7 +12,7 @@ init_intelclassic
 # The following compiler flags are used by the respective ICON configure
 # wrapper. They are not set into stone and can be changed if needed. It is just
 # important to keep them the same as in the wrapper.
-CFLAGS='-g -gdwarf-4 -qno-opt-dynamic-align -m64 -march=core-avx2 -mtune=core-avx2 -m64 -march=core-avx2 -mtune=core-avx2 -fma -ip -pc64 -std=gnu99 -O2 -ftz'
+CFLAGS='-g -gdwarf-4 -qno-opt-dynamic-align -m64 -march=core-avx2 -mtune=core-avx2 -fma -ip -pc64 -std=gnu99 -O2 -ftz'
 FCFLAGS='-m64 -march=core-avx2 -mtune=core-avx2 -g -gdwarf-4 -pc64 -fp-model source -O3 -ftz -qoverride-limits -assume realloc_lhs -align array64byte -fma -ip'
 
 if test ! -f "${top_srcdir}/configure"; then