glsl/glcpp: Don't choke on an empty pragma

glsl/glcpp: Don't choke on an empty pragma

The lexer was insisting that there be at least one character after "#pragma"
and before the end of the line. This caused an error for a line consisting
only of "#pragma" which volates at least the following sentence from the GLSL
ES Specification 3.00.4:

	The scope as well as the effect of the optimize and debug pragmas is
	implementation-dependent except that their use must not generate an
	error. [Page 12 (Page 28 of PDF)]

and likely the following sentence from that specification and also in
GLSLangSpec 4.30.6:

	If an implementation does not recognize the tokens following #pragma,
	then it will ignore that pragma.

Add a "make check" test to ensure no future regressions.

This change fixes at least part of the following Khronos GLES3 CTS test:

	preprocessor.pragmas.pragma_vertex

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