unset MAKELEVEL in scriptlets

[Pierre Labastie]

jhalfs (and blfs extension) run scriptlets from a Makefile, so that MAKELEVEL=1 when starting "make" in a build directory. This can lead to weird errors, as explained in postgresql INSTALL file:

If you want to invoke the build from another makefile rather than
       manually, you must unset MAKELEVEL or set it to zero, for instance
       like this:
build-postgresql:
        $(MAKE) -C postgresql MAKELEVEL=0 all

       Failure to do that can lead to strange error messages, typically
       about missing header files.

[Pierre Labastie]

I guess MAKELEVEL=1 is harmless in most cases, but for the few cases it is not (postgresql is one such case), we should unset MAKELEVEL (or set it to 0) in the generated scriptlets.

[Pierre Labastie]

Fixed at r4088