Add a switch to allow passing --gcov-tool to lcov (which can now be llvm-cov if using CLang) Refs #4008
store arg as string rather than list. References #4008
Merge pull request #4030 from milljm/coverage

build_coverage script can now handle clang coverage
Merge commit '0aebc7f015c2f5435a8bc006fbfef61211322b25'