Support V8 10.5

[stesie]

V8 10.5 uses c++17 features (is_lvalue_reference_v), hence we need to adapt our config process to detect c++17 and use that (opposed to c++14 like we did before)

With that configuration works, compilation seems to pass pretty far but in the end fails with

/app/v8js_main.cc: In function 'int zm_shutdown_v8js(int, int)':
/app/v8js_main.cc:165:11: error: 'ShutdownPlatform' is not a member of 'v8::V8'
  165 |   v8::V8::ShutdownPlatform();
      |           ^~~~~~~~~~~~~~~~
make: *** [Makefile:218: v8js_main.lo] Error 1

Needs to be "investigated" where that function went to :)

refs #489

[stesie]

/app/v8js_variables.cc: In function 'void v8js_register_accessors(std::vector<v8js_accessor_ctx*>*, v8::Local<v8::FunctionTemplate>, zval*, v8::Isolate*)':
/app/v8js_variables.cc:83:212: error: 'v8::AccessorSignature' has not been declared
   83 | ast<int>(ZSTR_LEN(property_name))), v8js_fetch_php_variable, NULL, v8::External::New(isolate, ctx), v8::PROHIBITS_OVERWRITING, v8::ReadOnly, v8::AccessorSignature::New(isolate, php_obj_t));
      |                                                                                                                                                  ^~~~~~~~~~~~~~~~~

make: *** [Makefile:230: v8js_variables.lo] Error 1

[stesie]

If you've compiled V8 in default configuration it'll have sandbox enabled. If so, you need to pass -DV8_ENABLE_SANDBOX to php-v8js as well. Like so:

./configure --with-v8js=/opt/libv8-10.5 LDFLAGS="-lstdc++" CPPFLAGS="-DV8_COMPRESS_POINTERS -DV8_ENABLE_SANDBOX"

[stesie]

With these changes php-v8js compiles just fine with V8 10.5.81 and also passes the test suite. Please note that this PR is based on php7 branch, hence I've tried PHP 7.4.3

This PR needs some more love, especially

• conditionally call either ShutdownPlatform or DisposePlatform, depending on V8 version
• mention the -DV8_ENABLE_SANDBOX flag in README (and maybe simply ignore it on older V8 version so it's safe to unconditionally pass it)
• auto-detect V8 sandbox and initialize it if needed

But feel free to already give it a try, give feedback and tell whether it works for you.

cc @temuri416