Reinstate genhtml --prefix option for non-vpath builds

Reinstate genhtml --prefix option for non-vpath builds

In c3d9a66024a93e6d0380bdd1b18cb03a67216b72, the genhtml --prefix option
was removed to get slightly better behavior for vpath builds.  genhtml
would then automatically pick a suitable prefix.  However, for non-vpath
builds, this makes the coverage output dependent on the length of the
path where the source code happens to be, leading to confusingly
arbitrary results.  So put the --prefix option back for non-vpath
builds.