Add -fcommon to autotools + meson (and fix some warnings)
While this does not fix the multiple definitions, it allows the code to compile "by default", so also outside of the CI.
Additionally, fix various compiler warnings.
NOTE: for autotools, this change means that the autoconf-archive package needs to be present because of the use of AX_APPEND_FLAG.
Signed-off-by: Johannes Kauffmann johanneskauffmann@hotmail.com