adding documentation for --enable-languages

[DJ Delorie]

Just ran across this in the mail archives?

> gcov     - gcov program (why gcov$(exeext) in gcc/Makefile.in?)

Because gcov is a program.  The other targets are .phony's.  On
dos-like platforms (djgpp, cygwin), real programs have an .exe suffix,
and the Makefile needs to reflect that.  The only way around that
would be for configure to add a .phony for gcov if $(exeext) is
non-null.