Bump C++ requirement to C++17

The 1.13.0 release of Poco requires a newer version of C++, specifically
C++17. The configuration phase will fail to find the Poco headers with
an older C++ version.

Changelog: https://raw.githubusercontent.com/pocoproject/poco/poco-1.13.0-release/CHANGELOG
Nixpkgs patch: https://github.com/NixOS/nixpkgs/blob/6dd5612cdd562163dc7a7341e3be9c3f6f4d9078/pkgs/tools/graphics/sanjuuni/default.nix#L22-L26
Relevant discussion: NixOS/nixpkgs#283809 (comment)

README.md

@@ -44,7 +44,7 @@ Nix support is maintained by [Tomodachi94](https://github.com/tomodachi94). For
 
 ## Building
 Requirements:
-* C++11 or later compiler
+* C++17 or later compiler
 * FFmpeg libraries
 * Poco (Foundation and Util required; Net/NetSSL optional)
 * zlib (usually required by Poco)

configure

@@ -2522,7 +2522,7 @@ ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
 
-CXXFLAGS="$CXXFLAGS -std=c++11"
+CXXFLAGS="$CXXFLAGS -std=c++17"
 
 
 
@@ -3808,7 +3808,7 @@ cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 $ac_cxx_conftest_cxx11_program
 _ACEOF
-for ac_arg in '' -std=gnu++11 -std=gnu++0x -std=c++11 -std=c++0x -qlanglvl=extended0x -AA
+for ac_arg in '' -std=gnu++11 -std=gnu++0x -std=c++17 -std=c++0x -qlanglvl=extended0x -AA
 do
   CXX="$ac_save_CXX $ac_arg"
   if ac_fn_cxx_try_compile "$LINENO"
@@ -3894,15 +3894,15 @@ ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ex
 ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
 
 
-{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C++ compiler accepts -std=c++11" >&5
-printf %s "checking whether C++ compiler accepts -std=c++11... " >&6; }
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether C++ compiler accepts -std=c++17" >&5
+printf %s "checking whether C++ compiler accepts -std=c++17... " >&6; }
 if test ${ax_cv_check_cxxflags___std_cpp11+y}
 then :
   printf %s "(cached) " >&6
 else $as_nop
 
   ax_check_save_flags=$CXXFLAGS
-  CXXFLAGS="$CXXFLAGS  -std=c++11"
+  CXXFLAGS="$CXXFLAGS  -std=c++17"
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
@@ -3929,7 +3929,7 @@ if test "x$ax_cv_check_cxxflags___std_cpp11" = xyes
 then :
   :
 else $as_nop
-  as_fn_error $? "C++ compiler does not support -std=c++11." "$LINENO" 5
+  as_fn_error $? "C++ compiler does not support -std=c++17." "$LINENO" 5
 fi
 
 

configure.ac

@@ -4,10 +4,10 @@ m4_include([m4/find_cxx.m4])
 m4_include([m4/find_cxx11.m4])
 
 AC_LANG([C++])
-CXXFLAGS="$CXXFLAGS -std=c++11"
+CXXFLAGS="$CXXFLAGS -std=c++17"
 AC_PROG_CC
 AC_PROG_CXX
-AX_CHECK_COMPILE_FLAG([-std=c++11], [], [AC_MSG_ERROR([C++ compiler does not support -std=c++11.])])
+AX_CHECK_COMPILE_FLAG([-std=c++17], [], [AC_MSG_ERROR([C++ compiler does not support -std=c++17.])])
 
 AC_CHECK_HEADERS([libavformat/avformat.h], [], [AC_MSG_ERROR([Could not find libavformat headers.])])
 AC_CHECK_HEADERS([libavcodec/avcodec.h], [], [AC_MSG_ERROR([Could not find libavcodec headers.])])