preload/posix: Add a POSIX_TRACE build flag

To make it easier to turn tracing on and off in the posix profile, this patch
adds a build-time flag for it.

It's only useful for debugging.
preload/posix: Make the objects depend on codegen.h

The objects depend on codegen.h, but the Makefile doesn't know this, so
they're not rebuild after codegen.h changes.

This patch fixes that by adding the file as a dependency in Makefile.
preload/run: Add a dry-run option to fiu-run

This patch adds a new option, -n, to make fiu-run show the command line
instead of executing the program.

It can be useful when you need to run things manually (for example, under
gdb).
libfiu 0.93