Portduino: specify C++ version and add link pthread (#5642)

arch/portduino/portduino.ini

@@ -34,7 +34,9 @@ build_flags =
   -Isrc/platform/portduino
   -DRADIOLIB_EEPROM_UNSUPPORTED
   -DPORTDUINO_LINUX_HARDWARE
+  -lpthread
   -lstdc++fs
   -lbluetooth
   -lgpiod
-  -lyaml-cpp
+  -lyaml-cpp
+  -std=c++17

variants/portduino-buildroot/platformio.ini

@@ -3,7 +3,6 @@ extends = portduino_base
 ; Optional libraries should be appended to `PLATFORMIO_BUILD_FLAGS`
 ; environment variable in the buildroot environment.
 build_flags = ${portduino_base.build_flags} -O0 -I variants/portduino-buildroot
-  -std=c++17
 board = buildroot
 lib_deps = ${portduino_base.lib_deps}
-build_src_filter = ${portduino_base.build_src_filter}
+build_src_filter = ${portduino_base.build_src_filter}