build system: add checks and wrapper macros for __attribute__((always_inline)) and __attribute__((unused))

also is a if...elif chain for the LaTeX checks (if something goes
wrong, this makes the first instead of the last issure appear in the
summary entry)

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7617 2fb0f335-1f38-0410-981e-8018bf24f1b0