Makefile: remove unnecessary setting of DESTDIR

Using an undefined variable is not an error -- the variable is simply
replaced by the empty string. The GNU Coding Standards concurs:

  You should not set the value of DESTDIR in your Makefile at all;
  then the files are installed into their expected locations by
  default. Also, specifying DESTDIR should not change the operation of
  the software in any way, so its value should not be included in any
  file contents.

  http://www.gnu.org/prep/standards/html_node/DESTDIR.html
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
Merge pull request #254
Merge pull request #254 from mgeisler/remove-destdir

Makefile: remove unnecessary setting of DESTDIR
Remove silent-by-default CC and CXX variables

I find it strange and annoying that 'make all' has no output. The
default behavior might be too verbose, but it's standard and deviating
too fra from it is not helpful.
Don't misuse headers for bold text.

Headers are not meant to be used for emphasis -- they define the
logical structure of the document. Bold and italic are the correct
tools to use when emphasizing text.
Wrap long lines

This makes the source easier to read in, say, a terminal of standard
width. The change does not affect the rendered output since Markdown
requires paragraphs need to be separated by an empty line.
Ignore editor backup files
Fix broken link to non-existing section

The links now point to the validator and toolchain repos where the
user can actually find proper installation instructions.
Wrap more long lines
Fix broken reference to non-existing section

The "Install System Prerequisites" section has apparently been moved
to the toolchain README.
Merge pull request #257 from mgeisler/master

Cleanup README
Merge pull request #255 from mgeisler/remove-silent-compilers

Remove silent-by-default CC and CXX variables
Link to toolchain README directly
Merge pull request #258 from mgeisler/master

Link to toolchain README directly