How to transfrom a.c dir1/b.c to -I. -I./dir1

[PRC]

Hi All,

There is a variable in my Makefile script, whose value is:
src = a.c dir1/b.c c.c dir2/d.c
I wonder how to transform its value to
-I. -I./dir1 -I. -I./dir2
in the Makefile script only using Make built-in functions, 
without invoking shell utilities (I think shell functions are slower than
built-in functions. Am I right? ).


Best Regards
PRC
Aug 1, 2008