Add a PlatformIO hook to alter CXXFLAGS

buildroot/share/PlatformIO/scripts/common-cxxflags.py

+#
+# common-cxxflags.py
+# Convenience script to apply customizations to CPP flags
+#
+Import("env")
+env.Append(CXXFLAGS=[
+  "-Wno-register"
+  #"-Wno-incompatible-pointer-types",
+  #"-Wno-unused-const-variable",
+  #"-Wno-maybe-uninitialized",
+  #"-Wno-sign-compare"
+])

platformio.ini

 
 [common]
 default_src_filter = +<src/*> -<src/config> -<src/HAL> +<src/HAL/shared>
+extra_scripts = pre:buildroot/share/PlatformIO/scripts/common-cxxflags.py
 build_flags = -fmax-errors=5
   -g
   -ggdb
 board         = adafruit_grandcentral_m4
 framework     = arduino
 build_flags   = ${common.build_flags} -std=gnu++17
+extra_scripts = ${common.extra_scripts}
 build_unflags = -std=gnu++11
 lib_deps      = ${common.lib_deps}
 lib_ignore    = U8glib-HAL