Compiler: Add __line

Imported from SA-MP: 605ae7f4d3
This is a backport from Pawn 4.0. It sets to the current line number during compile time.
Documention for 3.x actually mention this.

compiler/libpc300/sc1.c

@@ -1449,7 +1449,8 @@ static void setconstants(void)
   add_constant("ucharmax",(1 << (sizeof(cell)-1)*8)-1,sGLOBAL,0);
 
   add_constant("__Pawn",VERSION_INT,sGLOBAL,0);
-  
+  add_constant("__line", 0, sGLOBAL, 0);
+
   pc_anytag=pc_addtag("any");
 
   debug=0;