Build xtb and its dependencies with -Wall -Wextra without warnings

[foxtran]

Compilation of xtb with -Wall -Wextra gives a lot of warnings. Most of them are useless like unused variables and equality comparison of reals, but in some places they marks possible problems like

Warning: Possible change of value in conversion from REAL(8) to INTEGER(4) at

that may leads to bugs.

Compilation of xtb with GCC should be warnings-free.

At the first stage, compilation of xtb (and its deps) with -Wall -Wextra -Wno-unused-variable -Wno-maybe-uninitialized -Wno-unused-dummy-argument -Wno-unused-function -Wno-unused-parameter -Wno-compare-reals should not give any warnings. Later, -Wno- flags should be also removed.

[awvwgk]

GFortran has many false positives for -Wmaybe-uninitialized for allocatable variables with automatic left hand side allocation, therefore I would be careful with using -Wall -Wextra, but in principle I would agree that compilation should be warning free.

[foxtran]

Yep, that is why I suggest to skip this flag at the first stage