use std::pow instead of pow to fix solaris build

[eddelbuettel]

We seem to fail to build on Slowlaris as seen here.

This may help.

[adamjohansen]

Thanks Dirk, this looks pretty uncontroversial to me... fingers crossed that it resolves the ambiguity.

[eddelbuettel]

I tried it at r-hub in order to use solaris, but the builder had a network issue. Log is here.

[eddelbuettel]

Wow. @gaborcsardi already fixed the rhub issue I filed and I just checked the package with this PR there. No issue. (Now, I am not fully sure that it is the identical setup but probably close enough.

[eddelbuettel]

Hah! When I check the master branch (without the std::pow() change) it does indeed break.

So PR is good.

[adamjohansen]

Excellent... any reason not to merge?

[tibi77]

Why the pow from src/blockpfgaussianopt.cpp is not changed? Is there a reason that I am missing? (Asking from a consistency point of view).

[eddelbuettel]

@adamjohansen No, I'll do it later.
@tibi77 You can comment on the actual line. Some remain because they are in arma::pow(); the confusion (for the Solaris compiler) arose over std::pow()