Make C/C++/ObjC include directive scanning pattern more strict.

1. Require that the directive is at the beginning of a line, sans
   leading spaces.
2. Require that the header name is not empty.

This eliminates majority of comments that look like #include directives.
One practical example is a comment in Boost.Wave that has "#include <>"
and "#include """ in its body.

src/tools/types/cpp.jam

@@ -30,7 +30,7 @@ class c-scanner : scanner
 
     rule pattern ( )
     {
-        return "#[ \t]*include[ \t]*(<(.*)>|\"(.*)\")" ;
+        return "^[ \t]*#[ \t]*include[ \t]*(<(.+)>|\"(.+)\")" ;
     }
 
     rule process ( target : matches * : binding )

src/tools/types/objc.jam

@@ -14,7 +14,7 @@ class objc-scanner : c-scanner
 
     rule pattern ( )
     {
-        return "#[ \t]*include|import[ ]*(<(.*)>|\"(.*)\")" ;
+        return "^[ \t]*#[ \t]*include|import[ ]*(<(.+)>|\"(.+)\")" ;
     }
 }