MFC r283801,r290846,r290851,r290856,r290860:

r283801 (by araujo):

Fix warning of implicit declaration of function 'mkdir'.

Differential Revision: D2662
Reviewed by: rodrigc, ngie

r290846:

Bump WARNS to 2

Sponsored by: EMC / Isilon Storage Division

r290851:

Change WARNS to 2 across the board with all the libc testcases

This effectively "reverts" r290846

Sponsored by: EMC / Isilon Storage Division

r290856 (by bapt):

also skip the definition of ':fopen_regular' to avoid the build to fail due to
unused variables defined by ATF macros

r290860 (by bapt):

Remove unused variables to fix building world