gnu: Add a C++14 variant of Boost for packages that need it.

Fixes <https://bugs.gnu.org/33605> and <https://bugs.gnu.org/33610>.

* gnu/packages/boost.scm (boost-cxx14): New variable.
* gnu/packages/compression.scm (innoextract)[inputs]: Use it.
* gnu/packages/mpd.scm (ncmpcpp)[inputs]: Use it.

gnu/packages/boost.scm

@@ -122,6 +122,16 @@ (define-public boost
     (license (license:x11-style "https://www.boost.org/LICENSE_1_0.txt"
                                 "Some components have other similar licences."))))
 
+;; Some programs need Boost to be built with C++14 support.
+(define-public boost-cxx14
+  (package (inherit boost)
+    (arguments
+      (substitute-keyword-arguments (package-arguments boost)
+        ((#:make-flags flags)
+         `(append ,flags
+                  '("cxxflags=-std=c++14")))))
+    (properties '((hidden? . #t)))))
+
 (define-public boost-for-mysql
   ;; Older version for MySQL 5.7.23.
   (package

gnu/packages/compression.scm

@@ -2263,7 +2263,7 @@ (define-public innoextract
    (build-system cmake-build-system)
    (arguments
     `(#:tests? #f)) ;; No tests available.
-   (inputs `(("boost" ,boost)
+   (inputs `(("boost" ,boost-cxx14)
              ("libiconv" ,libiconv)
              ("xz" ,xz)))
    (native-inputs `(("pkg-config" ,pkg-config)))

gnu/packages/mpd.scm

@@ -244,7 +244,7 @@ (define-public ncmpcpp
                 "0m0mjb049sl62vx13h9waavysa30mk0rphacksnvf94n13la62v5"))))
     (build-system gnu-build-system)
     (inputs `(("libmpdclient" ,libmpdclient)
-              ("boost"  ,boost)
+              ("boost"  ,boost-cxx14)
               ("readline" ,readline)
               ("ncurses" ,ncurses)
               ("taglib" ,taglib)