More build-process updates, esp. for non-system case

Separate out LIBS from LDFLAGS Makefile variables, so that the LDFLAGS
can go before the LIBS.
Switch trace.sh to using the "-injection child" mode, which works
with the anti-ptrace security measures in many modern systems.
Add "include" and "lib" as generic paths for putting extra headers
and libraries that aren't in system locations.
Corresponding README updates.
With these changes I can build on an Ubuntu 12.04 64-bit system
completely as a regular user.