gnu: libmp4v2: Fix build with C++11.

* gnu/packages/patches/libmp4v2-c++11.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/video.scm (libmp4v2)[source](patches): New field.

gnu/local.mk

@@ -1023,6 +1023,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/libmad-armv7-thumb-pt2.patch		\
   %D%/packages/patches/libmad-frame-length.patch		\
   %D%/packages/patches/libmad-mips-newgcc.patch			\
+  %D%/packages/patches/libmp4v2-c++11.patch			\
   %D%/packages/patches/libmygpo-qt-fix-qt-5.11.patch		\
   %D%/packages/patches/libmygpo-qt-missing-qt5-modules.patch	\
   %D%/packages/patches/libreoffice-boost.patch			\

gnu/packages/patches/libmp4v2-c++11.patch

@@ -0,0 +1,14 @@
+Avoid a type mismatch that shows up in C++11.
+Taken from <https://bugs.debian.org/853560>.
+
+--- a/src/rtphint.cpp	2012-05-20 16:11:53.000000000 -0600
++++ b/src/rtphint.cpp	2017-05-16 10:25:26.930705191 -0600
+@@ -339,7 +339,7 @@
+                 pSlash = strchr(pSlash, '/');
+                 if (pSlash != NULL) {
+                     pSlash++;
+-                    if (pSlash != '\0') {
++                    if (*pSlash != '\0') {
+                         length = (uint32_t)strlen(pRtpMap) - (pSlash - pRtpMap);
+                         *ppEncodingParams = (char *)MP4Calloc(length + 1);
+                         strncpy(*ppEncodingParams, pSlash, length);

gnu/packages/video.scm

@@ -2746,6 +2746,7 @@ (define-public libmp4v2
                            "code-archive-downloads/v2/"
                            "code.google.com/mp4v2/mp4v2-" version ".tar.bz2"))
        (file-name (string-append name "-" version ".tar.bz2"))
+       (patches (search-patches "libmp4v2-c++11.patch"))
        (sha256
         (base32
          "0f438bimimsvxjbdp4vsr8hjw2nwggmhaxgcw07g2z361fkbj683"))))