gnu: Boost: Build with C++14.

Some parts of Boost which are used by other Guix packages require C++14 support.

Fixes <https://bugs.gnu.org/33605>.

* gnu/packages/boost.scm (boost)[arguments]: Pass 'cxxflags=-std=c++14'
to #:make-flags.

gnu/packages/boost.scm

@@ -67,6 +67,7 @@
      `(#:tests? #f
        #:make-flags
        (list "threading=multi" "link=shared"
+             "cxxflags=-std=c++14"
 
              ;; Set the RUNPATH to $libdir so that the libs find each other.
              (string-append "linkflags=-Wl,-rpath="