- Make doxygen pick up markdown.
- introduced: set_default_build_to_release.cmake

set_default_build_type_to_release.cmake -> set_default_build_to_release.cmake

'*.md' files put into the projects doc directory will be picked as part of
the documentation. '*.md' files in other locations of the source tree will
be ignored.

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.

fix issue that a double "-l" can appear in the Makefile config shell scripts for libraries which already contain the -l

usage:
register_tests(SOURCES
                 ${list_of_source_files}
                 "test_src.cc"
                 ${concatenated_string_of_source_files}
               NAMESPACE
                 "test_namespace"
               LINK_LIBRARIES
                 ${list_of_targets}
               INCLUDE_DIRECTORIES
                 ${list_of_include_directories}
               COMPILE_OPTIONS
                 ${list_of_compile_options}
               WORKING_DIRECTORY
                 ${ctest_working_directory})