Define the environment variable LTC_COVERAGE to something, run testme.sh
with all the coverage compile time options enabled and then 'make lcov'
creates the combined coverage for all combinations of compile-time
options handled in testme.sh.
e.g. LTC_COVERAGE=1 ./testme.sh "makefile -j3" "-DUSE_LTM -DLTM_DESC -I../libtommath -fprofile-arcs -ftest-coverage" "../libtommath/libtommath.a -lgcov"