Add -Wall -Wextra -Wpedantic (or -pedantic) warnings by default.

This also introduces a MIRALL_FATAL_WARNINGS to enable a -Werror for
GNU compilers and clang.

MSVC compilers are yet to be handled.
Merge pull request #836 from krnowak/warnings

Add -Wall -Wextra -Wpedantic (or -pedantic) warnings by default.