remove system/ansi_c include from os

lib/pure/os.nim

@@ -26,7 +26,6 @@ elif defined(posix):
 else:
   {.error: "OS module not ported to your operating system!".}
 
-include "system/ansi_c"
 include ospaths
 
 when defined(posix):
@@ -37,6 +36,21 @@ when defined(posix):
     var
       pathMax {.importc: "PATH_MAX", header: "<stdlib.h>".}: cint
 
+proc c_remove(filename: cstring): cint {.
+  importc: "remove", header: "<stdio.h>".}
+proc c_rename(oldname, newname: cstring): cint {.
+  importc: "rename", header: "<stdio.h>".}
+proc c_system(cmd: cstring): cint {.
+  importc: "system", header: "<stdlib.h>".}
+proc c_strerror(errnum: cint): cstring {.
+  importc: "strerror", header: "<string.h>".}
+proc c_strlen(a: cstring): cint {.
+  importc: "strlen", header: "<string.h>", noSideEffect.}
+proc c_getenv(env: cstring): cstring {.
+  importc: "getenv", header: "<stdlib.h>".}
+proc c_putenv(env: cstring): cint {.
+  importc: "putenv", header: "<stdlib.h>".}
+
 proc osErrorMsg*(): string {.rtl, extern: "nos$1", deprecated.} =
   ## Retrieves the operating system's error flag, ``errno``.
   ## On Windows ``GetLastError`` is checked before ``errno``.