Some USEMODULE parts (e.g. defined in ${BOARD}/Makefile.dep) were not
being properly exported to CFLAGS -DMODULE_xxx options.

If we use `rm -r(f)` then we should make sure that we don't delete
random directory trees because there was a whitespace in the path.

Remove space at begginning or error output line.

When invoking make with QUIET=0 these test are printed to stdout, which
could be confusing.

Instead of looking in the current directory for the application source
files, look into the defined APPDIR directory.

Fixes #1282.

Otherwise the shell will try to evaluate the parentheses.

OS X linker won't link archives consisting of the archives header only.

This allows having applications outside of RIOT's own git directory and
still get the correct branch information for the build.

Can be overridden from the environment by setting WERROR=0

also remove reversed logic for Werror in Makefile.buildtest
(a typo caused Werror to be always enabled for all builds)