RFC: Check IFUNC_XXX to enable/disable ifunc feature

[H.J. Lu]

Hi,

I'd like to check IFUNC_XXX environmental variable to enable/disable
the ifunc feature, XXX.  That is

1.  IFUNC_XXX=1 to enable XXX.
2.  IFUNC_XXX=0 to disable XXX.

Any comments?

-- 
H.J.