AC_TDD_GCOV: Get rid of use_gcov.

AC_ARG_ENABLE was not being used correctly: its last parameter is "action-if-not-given", not "action-if-disabled-is-passed", so the only way to actually make autotools not look for gcov/lcov/etc was to not pass --enable-gcov or --disable-gcov to the command line; in other words, --disable-gcov also set use_gcov to "yes" and did the opposite of what it was supposed to do.

Since AC_ARG_ENABLE automatically sets the enable_$feature variable with the values we wanted use_gcov to have, just use that one instead.