glsl/glcpp: Add short-circuiting for || and && in #if/#elif for OpenGL ES.

glsl/glcpp: Add short-circuiting for || and && in #if/#elif for OpenGL ES.

The GLSL ES Specification 3.00.4 says:

	#if, #ifdef, #ifndef, #else, #elif, and #endif are defined to operate
        as for C++ except for the following:
	...
	• Undefined identifiers not consumed by the defined operator do not
	  default to '0'. Use of such identifiers causes an error.

	[Page 11 (page 127 of the PDF file)]

as well as:

	The semantics of applying operators in the preprocessor match those
	standard in the C++ preprocessor with the following exceptions:

	• The 2nd operand in a logical and ('&&') operation is evaluated if
	  and only if the 1st operand evaluates to non-zero.

	• The 2nd operand in a logical or ('||') operation is evaluated if
	  and only if the 1st operand evaluates to zero.

	If an operand is not evaluated, the presence of undefined identifiers
	in the operand will not cause an error.

(Note that neither of these deviations from C++ preprocessor behavior apply to
non-ES GLSL, at least as of specfication version 4.30.6).

The first portion of this, (generating an error for an undefined macro in an
(short-circuiting to squelch errors), was not implemented previously, but is
implemented in this commit.

A test is added for "make check" to ensure this behavior.

Note: The change as implemented does make the error message a bit less
precise, (it just states that an undefined macro was encountered, but not the
name of the macro).

This commit fixes the following Khronos GLES3 conformance test:

	undefined_identifiers.valid_undefined_identifier_1_vertex
	undefined_identifiers.valid_undefined_identifier_1_fragment
	undefined_identifiers.valid_undefined_identifier_2_vertex
	undefined_identifiers.valid_undefined_identifier_2_fragment

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>