libvisual: Require a C++11 compiler + enable C++11 features

Related documentation:
https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html

Author: hartwork

.github/workflows/enforce-copies-of-help-output-in-sync.yml

@@ -35,6 +35,7 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install --yes --no-install-recommends \
+            autoconf-archive \
             autopoint \
             gettext \
             help2man \

.github/workflows/linux_and_macos.yml

@@ -65,6 +65,7 @@ jobs:
           #       >                             is to be installed
           sudo apt-get install --yes --no-install-recommends libunwind-dev
           sudo apt-get install --yes --no-install-recommends \
+            autoconf-archive \
             autopoint \
             bison \
             doxygen \
@@ -93,6 +94,7 @@ jobs:
         if: runner.os == 'macOS'
         run: |-
           brew install \
+            autoconf-archive \
             automake \
             bison \
             doxygen \

.github/workflows/translations.yml

@@ -23,6 +23,7 @@ jobs:
           #       >                             is to be installed
           sudo apt-get install --yes --no-install-recommends libunwind-dev
           sudo apt-get install --yes --no-install-recommends \
+            autoconf-archive \
             autopoint \
             bison \
             doxygen \

libvisual/configure.ac

@@ -112,6 +112,7 @@ AC_HEADER_DIRENT
 AC_CHECK_HEADERS([fcntl.h sys/types.h stdlib.h string.h sys/time.h unistd.h sched.h sys/sched.h])
 
 # Checks for typedefs, structures, and compiler characteristics.
+AX_CXX_COMPILE_STDCXX_11([noext], [mandatory])
 AC_C_CONST
 AC_C_INLINE
 AC_TYPE_SIZE_T