meson: Ensure that abseil is built with c++17 too

subprojects do not inherit $lang_std default values from the project.

meson.build

@@ -55,7 +55,7 @@ have_win = false
 # if pkg-config is not found, which is really the most reliable way of building
 # abseil due to strict C++ standard match requirements.
 absl_dep = [
-  dependency('absl_base'),
+  dependency('absl_base', default_options: ['cpp_std=c++17']),
   dependency('absl_flags'),
   dependency('absl_strings'),
   dependency('absl_synchronization'),