Fix bug causing some #undefs to be ignored when using a sym file.

This would occur if the macro had already been saved in the sym file and the #undef occurred before a subsequent #include that was also recorded in the sym file. The solution is simply to terminate sym file generation if an #undef of an already-saved macro is encountered.

Here is an example showing the problem:

test.c:
#include "test1.h"
#undef x
#include "test2.h"

int main(void) {
#ifdef x
        return x;
#else
        return y;
#endif
}

test1.h:
#define x 27

test2.h:
#define y 6

Header.pas

@@ -18,7 +18,7 @@ uses CCommon, MM, Scanner, Symbol, CGI;
 {$segment 'SCANNER'}
 
 const
-   symFileVersion = 21;                 {version number of .sym file format}
+   symFileVersion = 22;                 {version number of .sym file format}
 
 var
    inhibitHeader: boolean;		{should .sym includes be blocked?}

Scanner.pas

@@ -2943,6 +2943,8 @@ var
                   bPtr^ := mPtr^.next
                else
                   lastPtr^.next := mPtr^.next;
+               if mPtr^.saved then
+                  TermHeader;
                end; {else}
             goto 1;
             end; {if}

cc.notes

@@ -1775,6 +1775,8 @@ int foo(int[42]);
 
 179. Macro definitions or header search paths specified via the cc= portion of the command line could be saved in the .sym file and applied to subsequent compilations, even if they were no longer specified on the command line.
 
+180. Certain #undef directives might be ignored when using a .sym file.
+
 -- Bugs from C 2.1.0 that have been fixed -----------------------------------
 
 1.  In some situations, fread() reread the first 1K or so of the file.