fix warnings with GCC 9.2 and -Wextra and some cleanup
Compiling OPAL with GCC 9.2 still gives some warnings if -Wextra is enabled.
Enable -Werror for all compiler (which was once the default anyway).
Compiling OPAL with GCC 9.2 still gives some warnings if -Wextra is enabled.
Enable -Werror for all compiler (which was once the default anyway).