include: Add macro to ignore -Wunused-but-set-variable

"-Wunused-but-set-variable" warning option can be helpful to track variables
that are written but not read thereafter.  But it can be harmful if some of
the code is auto-generated and we have no ways to deal with it.

The particular example is Bison-generated code.

The new DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE macro can be helpful on
such cases. A typical use of this macro is to place this macro before the
end of user prologues on Bison (.y) files.

include/ChangeLog:

    * diagnostics.h (DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE): New.

include/diagnostics.h

@@ -68,6 +68,11 @@
    DIAGNOSTIC_IGNORE ("-Wuser-defined-warnings")
 # endif
 
+# if __has_warning ("-Wunused-but-set-variable")
+#  define DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE \
+   DIAGNOSTIC_IGNORE ("-Wunused-but-set-variable")
+# endif
+
 # define DIAGNOSTIC_ERROR_SWITCH \
   DIAGNOSTIC_ERROR ("-Wswitch")
 
@@ -89,6 +94,11 @@
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL \
   DIAGNOSTIC_IGNORE ("-Wformat-nonliteral")
 
+# if __GNUC__ >= 5
+#  define DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE \
+   DIAGNOSTIC_IGNORE ("-Wunused-but-set-variable")
+# endif
+
 /* GCC 4.8's "diagnostic push/pop" seems broken when using this, -Wswitch
    remains enabled at the error level even after a pop.  Therefore, don't
    use it for GCC < 5.  */
@@ -130,6 +140,10 @@
 # define DIAGNOSTIC_IGNORE_USER_DEFINED_WARNINGS
 #endif
 
+#ifndef DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE
+# define DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE
+#endif
+
 #ifndef DIAGNOSTIC_ERROR_SWITCH
 # define DIAGNOSTIC_ERROR_SWITCH
 #endif