glcpp -- GLSL "C" preprocessor

This is a simple preprocessor designed to provide the preprocessing
needs of the GLSL language. The requirements for this preprocessor are
specified in the GLSL 1.30 specification availble from:

http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.30.10.pdf

This specification is not precise on some semantics, (for example,
#define and #if), defining these merely "as is standard for C++
preprocessors". To fill in these details, I've been using a draft of
the C99 standard as available from:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

Any downstream compiler accepting output from glcpp should be prepared
to encounter and deal with the following preprocessor macros:

	#line
	#pragma
	#extension

All other macros will be handles according to the GLSL specification
and will not appear in the output.

Known limitations
-----------------
The __LINE__ and __FILE__ macros are not yet supported.

A file that ends with a function-like macro name as the last
non-whitespace token will result in a parse error, (where it should be
passed through as is).

Known deviations from the specification
---------------------------------------
As mentoned above, the GLSL specification (as of 1.30.10) is fairly
vague on some aspects of the preprocessor, and we've been using C99 to
fill in details. Here is a list of cases where we have deviated from
the behavior specified in C99 to obtain better compatibility with
other GLSL implementations:

  * Redefining a macro with a different value

    C89 says that a macro "may be redefined ... provided that the
    second definition [is equivalent]" (Section 3.8.3 Macro
    Replacement/constraints)

    C99 is even more explicit, saying tthat a macro "shall not be
    redefined by another #define preprocessing directive unless the
    second definition [is equivalent]" (Section 6.10.3 Macro
    Replacement/Constraints)

    In spite of this, glcpp emits a warning rather than an error for
    non-equivalent redefinition of macros since this matches the
    behavior of other, widely-used implementations.