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>

src/glsl/glcpp/glcpp-lex.l

@@ -180,7 +180,7 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 
 	/* glcpp doesn't handle #extension, #version, or #pragma directives.
 	 * Simply pass them through to the main compiler's lexer/parser. */
-{HASH}(extension|pragma)[^\n]+ {
+{HASH}(extension|pragma)[^\n]* {
 	if (parser->commented_newlines)
 		BEGIN NEWLINE_CATCHUP;
 	yylval->str = ralloc_strdup (yyextra, yytext);

src/glsl/glcpp/tests/127-pragma-empty.c

@@ -0,0 +1,3 @@
+/* It seems an odd (and particularly useless) thing to have an empty pragma,
+ * but we probably shouldn't trigger an error in this case. */
+#pragma

src/glsl/glcpp/tests/127-pragma-empty.c.expected

@@ -0,0 +1,4 @@
+ 
+
+#pragma
+