gnu: Remove more GCC < 7 workarounds.

* gnu/packages/abiword.scm (abiword)[arguments]: Do not pass "-std=c++11" to
the GCC command line.
* gnu/packages/aidc.scm (zbar)[arguments]: Likewise.
* gnu/packages/android.scm (android-libbase, android-libcutils,
android-libziparchive, fastboot)[arguments]: Likewise.
* gnu/packages/audio.scm (azr3, guitarix, suil)[arguments]: Likewise.
* gnu/packages/bioinformatics.scm (bless)[arguments]: Likewise.
* gnu/packages/bittorrent.scm (libtorrent-rasterbar)[arguments]: Likewise.
* gnu/packages/code.scm (rtags)[arguments]: Likewise.
* gnu/packages/coq.scm (coq-gappa)[arguments]: Likewise.
* gnu/packages/emulators.scm (dolphin-emu)[arguments]: Likewise.
* gnu/packages/engineering.scm (xyce-serial, xyce-parallel)[arguments]: Likewise.
* gnu/packages/gnupg.scm (pinentry-qt)[arguments]: Likewise.
* gnu/packages/gobby.scm (libnet6, obby)[arguments]: Likewise.
* gnu/packages/graph.scm (python-faiss)[arguments]: Likewise.
* gnu/packages/graphics.scm (povray)[arguments]: Likewise.
* gnu/packages/image-processing.scm (opencv, itk-snap)[arguments]: Likewise.
* gnu/packages/image.scm (freeimage)[arguments]: Likewise.
* gnu/packages/kodi.scm (crossguid)[arguments]: Likewise.
* gnu/packages/music.scm (amsynth, qmidiarp, qmidiroute, seq24)[arguments]: Likewise.
* gnu/packages/pdf.scm (python-poppler-qt5)[arguments]: Likewise.
* gnu/packages/sagemath.scm (lcalc)[arguments]: Likewise.
* gnu/packages/video.scm (vlc, mlt, v4l-utils)[arguments]: Likewise.

gnu/packages/code.scm

@@ -431,7 +431,6 @@ functionality such as HTML output.")
      '(#:build-type "RelWithDebInfo"
        #:configure-flags
        '("-DRTAGS_NO_ELISP_FILES=1"
-         "-DCMAKE_CXX_FLAGS=-std=c++11"
          "-DBUILD_TESTING=FALSE")
        #:tests? #f))
     (native-inputs