ext/intl: fix clang build.

[devnexen]

actually, with recent icu releases, this problem arises

checking whether clang++ supports C++17 features with -std=c++17... no
checking whether clang++ supports C++17 features with +std=c++17... no
checking whether clang++ supports C++17 features with -h std=c++17... no
checking whether clang++ supports C++17 features with -std=c++1z... no
checking whether clang++ supports C++17 features with +std=c++1z... no
checking whether clang++ supports C++17 features with -h std=c++1z... no
configure: error: *** A compiler with support for C++17 language features is required.

thus, proposing to falls into LLVM libc++ instead.

[devnexen]

note it happens only when ubsan is enabled.

[devnexen]

so intl itself might be compiled with rtti however we compile c++ w/o exceptions nor rtti (lean binaries ?) so had to disable vptr sanitizer check.