add `make distcheck` to ci

currently, uninstalling does not remove all the files and make distcheck fails:

...
Must remake target `distuninstallcheck'.
Invoking recipe from Makefile:1185 to update target `distuninstallcheck'.
make[1]: Entering directory `/builds/***/ftimings/build/ftimings-0.2/_build/sub'
ERROR: files left after uninstall:
./lib/libftimings_perflib-1-gfortran.so.1.0.0
./lib/libftimings-1-gfortran.a
./lib/libftimings-1-gfortran.so.1.0.0
./lib/libftimings_perflib-1-gfortran.a
./include/ftimings-1-gfortran/x__ftimings_perf_wrapper_module.mod
./include/ftimings-1-gfortran/ftimings.mod
./include/ftimings-1-gfortran/ftimings_type.mod
./include/ftimings-1-gfortran/ftimings_value.mod
make[1]: *** [distuninstallcheck] Error 1
...

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