tccgen.c: Give error if statement expression found when const wanted.

Some test cases:

#define SE ({ switch (0) { } 0; })

// Should give error:
int x = SE;
void f(void) { static int x = SE; }
void f(void) { enum e { a = SE }; }
void f(void) { switch (0) { case SE: break; } }

// Correct:
int f(void) { return SE; }
int f(void) { return sizeof(SE); }

tccgen.c

@@ -3831,12 +3831,8 @@ ST_FUNC void unary(void)
                 gen_cast(&type);
             }
         } else if (tok == '{') {
-    	    /*
-            if (nocode_wanted)
-                tcc_error("statement expression in global scope"); */
-	    /* this check breaks compilation of the linux 2.4.26 with the meesage:
-	       linux/include/net/tcp.h:945: error: statement expression in global scope */
-
+            if (const_wanted)
+                tcc_error("expected constant");
             /* save all registers */
             save_regs(0); 
             /* statement expression : we do not accept break/continue