This adds variable _EXTRA_DEPENDS that is passed as direct dependency of
registered target.

If user defines COV variable in environment, then C files are built
with gcov.

Always execute "make clean" before using "COV" variable when it wasn't
used in previous call to make.

As they are not really standards. So fall back to the c/c++ ones, as the
gnu only adds some library calls, not syntax features.

• Extra comma in EXTRA_DEFINES expansion
• Better standards
• Better output

Use the FORCE target around cppcheck tasks. The FORCE target, while less
explicit about what it is, acts sane around pattern-based recipes.

Alloca is completely OK and we have reasons to use it. So allow it
globally. But it seems cppcheck is not consistent between versions in
its ID, so list both.

The cppcheck target (and its subtargets) for static code analysis.

Because they may be needed by our own libraries.

• Remove duplicate flags from the compilation (include $(2)_CFLAGS and
  $(2)_LDFLAGS only once in the command)
• Typo in STATIC_LOCAL variable name, preventing static linking from
  working.