include with absolute unix path not found on MinGW MINGW*

int h;

#include "/s/GitHub/simplecpp-fw/a.h"

int i;

$  ./simplecpp -l a.cpp
1:
2:
3: int i ;
a.cpp:1: missing header: Header not found: "/s/GitHub/simplecpp-fw/a.h"

$ ls -l /s/GitHub/simplecpp-fw/a.h
-rw-r--r-- 1 User None 6 Oct 13 21:25 /s/GitHub/simplecpp-fw/a.h

$  ./simplecpp -l a.cpp

#line 1 "s:/GitHub/simplecpp-fw/a.h"
1: int h ;
#line 3 "a.cpp"
3: int i ;