R-rstan: fix older systems with libc++

[barracuda156]

Description

Fix this finally too.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.8.5
Xcode 5.1.1

Verification

Have you

• followed our Commit Message Guidelines?
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• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[barracuda156]

P. S. The fix itself works fine and solves the problem, but it is suboptimal in a sense we will need to patch every dependent of RcppParallel in a same manner to fix them on older Intel systems.

Issue with upsteam: RcppCore/RcppParallel#219

[barracuda156]

I want to fix this from RcppParallel side instead of patching multiple dependents, therefore closing this now.