gnu: mash: Use C++ 14.

I'm looking to upgrade capnproto, and mash fails to build with 0.7. Therefore,
tweak the compilation to allow it to build with 0.7. The package also builds
with the current version of capnproto. I got the idea of changing the c++
version from here [1].

1: https://github.com/marbl/Mash/issues/98

* gnu/packages/bioinformatics.scm (mash)[arguments]: Add new use-c++14 phase.

gnu/packages/bioinformatics.scm

@@ -4024,6 +4024,14 @@ sequences).")
                             "src/mash/CommandScreen.cpp")
                (("^#include \"kseq\\.h\"")
                 "#include \"htslib/kseq.h\""))
+             #t))
+         (add-after 'fix-includes 'use-c++14
+           (lambda _
+             ;; capnproto 0.7 requires c++14 to build
+             (substitute* "configure.ac"
+               (("c\\+\\+11") "c++14"))
+             (substitute* "Makefile.in"
+               (("c\\+\\+11") "c++14"))
              #t)))))
     (native-inputs
      `(("autoconf" ,autoconf)