Implement token pasting as per the C++ specification, within the current
style of the PP code.
Non-identifiers (turning 12 ## 10 into the numeral 1210) is not yet covered;
they should be a simple incremental change built on this one.
Addresses issue #255.

The gtest executable accepts a --test-root option to specify
a root directory for test files.  It defaults to the Test directory
in the source tree from which the executable is built.

For example, this lets us run test exectuables built with MinGW on Linux
on a Windows machine with its own copy of the source tree.

This would look ahead for a second #, for token pasting, and if not
found, backup one token.  This is fine, unless at the end of line,
which would backup the #, rather than the look ahead.

This is used by OpenGL, but not Vulkan.
Includes:
 - atomicCounter, atomicIncrement, atomicCounterDecrement
 - atomic_uint layout-offset checking
 - AtomicStorage capability

Also beefed up support for running compute shaders is #version 420, but this
work is only partially done.

The existing test harness is a homemade shell script. All the tests
and the expected results are written in plain text files. The harness
just reads in a test, invoke the glslangValidator binary on it, and
compare the result with the golden file. All tests are kinda
integration tests.

This patch add Google Test as an external project, which provides a
new harness for reading shader source files, compile to SPIR-V, and
then compare with the expected output.