Bugfix change from function to macro

This change is similar to commit 89be4e14: After find_package_append_to
changed from function to macro to pick up the configuration not only
from the module itself but also from everything it pulled it, the
variable MODULE is overwritten (variable module in lower case is a
parameter, so it is replaced in the source body). Thus, the test in the
end is not whether *this* module was found, but if its last dependency
was! This made the build crash only in some projects but not in others.
Merge pull request #20 from rolk/020_tinyxml

Similar to OPM/opm-core#205