remove last reference of BUILD_NET
physics: remove false attribute
cmake: do not set -Werror for release builds

Depending on the compiler and the compiler version, more/less warnings
can be found. It means that someone wanting to make a release build of
Drystal with a compiler finding a warning that we do not have with our
compiler will not be able to build Drystal. Hence, disabling -Werror
for release builds.
cmake: fix indentation
util: remove useless function
util: add mkdir_p

Equivalent of mkdir -p

It replaces mkpath from main.cpp. This function could be reused later
elsewhere and it handles errors.
main: remove const attribute for argv

argv is not const, hence we don't need the const attribute