[Beignet] pow(n), erf(c), tgamma give wrong results

[Rebecca N. Palmer]

(Referring to 
https://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;filename=Fix-pow-erf-tgamma.patch;att=3;bug=768090 
)

> And it triggers some other bugs related to constant expression
> handling when run all unit test cases.
I didn't see that when I ran the tests in 0.9.3 on IvyBridge M GT2 (only 
builtin_pow and builtin_tgamma failed, both due to the absolute error 
checking you've already fixed) but your patch says the problem does 
exist in 0.9.x: how do I test for this bug?  Debian testing is still on 
0.8, and can't change to 0.9 now due to freeze rules.

> As to the license issue, we will fix it to a consistent version.
> I think change all statement to LGPL2.1+ should be ok.
The lack of a Signed-off-by line was solely due to the license issue (I 
don't know if there even is a difference between 2.0 and 2.1 other than 
the name change from Library to Lesser GPL), so given the above:

Signed-off-by: Rebecca Palmer <rebecca_palmer at zoho.com>