Use LDFLAGS when compiling zerovm

This lets the compiler find libvalidator correctly when it is
installed under PREFIX/lib and zerovm is compilsed using the same
PREFIX setting.

This removes the "need" to run 'sudo make install' when installing the
validator, e.g., the library can be installed in a custom location be
found without further hacking of the zerovm Makefile.
Makefile: use bindir and nacl_includedir variables
Merge pull request #253 from mgeisler/use-ldflags

Use LDFLAGS