cleanup_pkg() in tools:::.build_packages() is broken

Hi,

The cleanup_pkg() function defined the big tools:::.build_packages()
function in tools/R/build.R is currently broken. When Makefiles are
used cleanup_pkg() doesn't clean anything because of the way system2()
is called.

For example, the call to Ssystem() on line 304 (Ssystem is a silent
version of system2) is basically trying to do this:

 > system2(paste(Sys.getenv("MAKE", "make"), makefiles, "clean"))

sh: make -f '/home/hpages/R-2.12/etc//Makeconf' -f Makefile clean: not
found

But it seems that with this new system2(), the arguments to the system
command need to be passed separately thru the 'args' argument.

Cheers,
H.