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/abiword.scm

@@ -83,7 +83,7 @@
         ;; the tests to pass.
         #:tests? #f
         #:make-flags
-        (list "CXXFLAGS=-std=c++11" "gtk_update_icon_cache=true")))
+        (list "gtk_update_icon_cache=true")))
     (inputs
       `(("boost" ,boost)
         ("enchant" ,enchant)