Support for gprof-enabled modules

When a program is compiled and linked with ocamlopt, gprof-style profiling can be enabled by selecting the -p option. This means that the resulting archives are different, and that it would be useful if there were another predicate in order to select such archives.

The predicate "gprof" can be used for this. It is automatically set if the -p option turns on the gprof-mode in an ocamlopt invocation.