- Remove Makefile.coverage.in
 - Drop BUILD_COVERAGE_ON_DEBUG for ENABLE_COVERAGE_REPORT (defaults to
    on if included)
 - Add clean-gcda target

enclose CMAKE_BUILD_TYPE in quotes; this transforms it into an wmpty
string and the string command wont complain that its missing an argument
when CMAKE_BUILD_TYPE is not set.

Cmake treats CMAKE_BUILD_TYPE string as case insensitve. Make the check
for enabling coverage reflect this to make usage less restrictive.