Don't explicitly time bench.chap5* targets.

The `bench' procedure in chap5-bench.scm is already recording
runtimes. There is no need to also time the execution of the shell
command.
Increase the benchmark prime range from 50 to 500.

Even the slowest chap5a interpreter was reporting 0 seconds of elapsed
time with a max of 50. Increasing the prime range to 500 results in
runtimes on my ~4 year old laptop for bench.chap5a that are comparable
to the times listed at the bottom of chap5-bench.scm, which are probably
from ~20 years ago.

At any rate, increasing the range to 500 is sufficient to show a large
difference between the runtime for bench.chap5a (~53 seconds) and
bench.chap6a (~12 seconds).
Comment out the test.chap6a.bgl target.

It depends on a file, bigloo/compapp.scm, which was not included in the
source tarball from the author's website.
Fix warning -Wcomment when compiling rt.o.
Fix warning -Wparenthesis when compiling rt.o.
Fix warning -Wformat= when compiling rt.o.
Fix warning -Wformat= when compiling rt.o.

Format pointers using the %p directive.
Fix warning about extra semi-colons when compiling rt.o.
Fix warning -Wunused-value when compiling rt.o.

Remove unnecessary initialization clause from for-loop.
Fix warning -Wmissing-braces when compiling rt.o.

Fix missing-braces warning in the initialization of SCM_frame.
Fix warning -Wreturn-type when compiling rt.o.
Fix warning -Wint-to-pointer-cast when compiling rt.o.

Don't attempt to stuff a pointer into an int and pass it as the return
value of longjmp. Declare a static global to hold the return value.
Fix warning about converting function ptr to object ptr.
Fix warning -Wpointer-arith when compiling rt.o.

Use char * pointers instead of void * when doing pointer arithmetic to
compute the stack size.
Fix warning -Wunused-variable when compiling rt.o.

Initialize zone_static_end in staticize_allocator.
Fix warning -Wreturn-type when compiling chap6f.c.
Fix the bench.chap6f target.