glcpp: Demote "macro redefined" from an error to a warning

The GLSL specification is vague here, (just says "as is standard for
C++"), though the C specifications seem quite clear that this should
be an error.

However, an existing piglit test (CorrectPreprocess11.frag) expects
this to be a warning, not an error, so we change this, and document in
README the deviation from the specification.

src/glsl/glcpp/README

@@ -30,3 +30,26 @@ 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.

src/glsl/glcpp/glcpp-parse.c

@@ -4012,7 +4012,7 @@ _define_object_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
 			       identifier);
 	}
 
@@ -4043,7 +4043,7 @@ _define_function_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
 			       identifier);
 	}
 

src/glsl/glcpp/glcpp-parse.y

@@ -1687,7 +1687,7 @@ _define_object_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
 			       identifier);
 	}
 
@@ -1718,7 +1718,7 @@ _define_function_macro (glcpp_parser_t *parser,
 			talloc_free (macro);
 			return;
 		}
-		glcpp_error (loc, parser, "Redefinition of macro %s\n",
+		glcpp_warning (loc, parser, "Redefinition of macro %s\n",
 			       identifier);
 	}
 

src/glsl/glcpp/tests/089-redefine-macro-error.c.expected

@@ -1,14 +1,14 @@
-0:2(9): preprocessor error: Redefinition of macro x
+0:2(9): preprocessor warning: Redefinition of macro x
 
-0:5(9): preprocessor error: Redefinition of macro abc
+0:5(9): preprocessor warning: Redefinition of macro abc
 
-0:8(9): preprocessor error: Redefinition of macro foo
+0:8(9): preprocessor warning: Redefinition of macro foo
 
-0:11(9): preprocessor error: Redefinition of macro bar
+0:11(9): preprocessor warning: Redefinition of macro bar
 
-0:14(9): preprocessor error: Redefinition of macro biff
+0:14(9): preprocessor warning: Redefinition of macro biff
 
-0:17(9): preprocessor error: Redefinition of macro oper
+0:17(9): preprocessor warning: Redefinition of macro oper