Test for compiling without warnings against public headers

Under compilers with WERRORFLAG, MakeMakefile.try_compile treats
warnings as errors, so we can use append_cflags to test whether the
public header files emit warnings with certain flags turned on.

[Regression test for feature #20507]

ext/-test-/public_header_warnings/extconf.rb

@@ -0,0 +1,23 @@
+#
+#  Under compilers with WERRORFLAG, MakeMakefile.try_compile treats warnings as errors, so we can
+#  use append_cflags to test whether the public header files emit warnings with certain flags turned
+#  on.
+#
+def check_append_cflags(flag, msg = nil)
+  msg ||= "flag #{flag} is not acceptable"
+  !$CFLAGS.include?(flag) or raise("flag #{flag} already present in $CFLAGS")
+  append_cflags(flag)
+  $CFLAGS.include?(flag) or raise(msg)
+end
+
+if %w[gcc clang].include?(RbConfig::CONFIG['CC'])
+  config_string("WERRORFLAG") or raise("expected WERRORFLAG to be defined")
+
+  # should be acceptable on all modern C compilers
+  check_append_cflags("-D_TEST_OK", "baseline compiler warning test failed")
+
+  # Feature #20507: Allow compilation of C extensions with -Wconversion
+  check_append_cflags("-Wconversion", "-Wconversion raising warnings in public headers")
+end
+
+create_makefile("-test-/public_header_warnings")