This commit introduces two new options to CTestCoverageCollectGCOV.

When GLOB is set we recursively search in the source & binary
directories for .gcda files.  Otherwise the default behavior is to
parse TargetDirectories.txt for a list of locations to search.

When DELETE is set we remove any .gcda file found after it has
been used to generate the corresponding .gcov file.  The .gcov
file is also removed after the result tarball has been created.

Together these two new features help support the use case of
computing coverage across subprojects.