Add warning suppressions for external libraries in 'pio check'.

Addresses issue PlummersSoftwareLLC#622.

platformio.ini

@@ -44,6 +44,10 @@ build_flags     = -std=gnu++2a
 build_src_flags = -Wformat=2                        ; Warnings for our code only, excluding libraries
                   -Wformat-truncation
                   -Wstack-usage=4096
+
+# Exclude libraries that we don't control from 'pio check'.
+# format is error number(optional :path_with_wildcards(optional :line_number))
+check_flags     = cppcheck: --suppress=*:*src/uzlib/src/*
 
 lib_deps        = crankyoldgit/IRremoteESP8266  @ ^2.7.20
                   fastled/FastLED               @ ^3.4.0