libvpx @1.13.0: requires c++11

[kencu (Ken)]

libvpx already has various compiler blacklisting in the Portfile that approximates a c++11 standard, but it doesn't explicitly require c++11 so the proper MacPorts flags and libraries are not chosen on older systems:

/opt/local/bin/clang-mp-7.0  -pipe -Os -isysroot/ -arch i386 -DNDEBUG -O3 -fPIC -Wall -Wdeclaration-after-statement -Wdisabled-optimization -Wextra-semi -Wfloat-conversion -Wformat=2 -Wparentheses-equality -Wpointer-arith -Wtype-limits -Wcast-qual -Wvla -Wimplicit-function-declaration -Wmissing-declarations -Wmissing-prototypes -Wuninitialized -Wunreachable-code-loop-increment -Wunused -Wextra -Wundef -Wframe-larger-than=52000 -std=gnu89 -Wshorten-64-to-32 -I. -I"/opt/local/var/macports/build/_opt_macports-ports_multimedia_libvpx/libvpx/work/libvpx-1.13.0" -c -o md5_utils.c.o md5_utils.c
In file included from vp9/ratectrl_rtc.cc:10:
./vp9/ratectrl_rtc.h:14:10: fatal error: 'cstdint' file not found
#include <cstdint>
         ^~~~~~~~~
1 error generated.
make[1]: *** [vp9/ratectrl_rtc.cc.o] Error 1
make[1]: *** Waiting for unfinished jobs....

However, to match the current Portfile blacklisting, some extra blacklisting will still have to be retained in the Portfile.