C++ feature checks: Filter out libstdc++ sprintf warnings

On OpenBSD linking to `libstdc++` with GCC 6.4 always warns:

    warning: sprintf() is often misused, please use snprintf()

These do not affect the availability of C++ features we're checking,
so filter them out.

Fixes: #18602

Source/Checks/cm_cxx_features.cmake

@@ -22,6 +22,8 @@ function(cm_check_cxx_feature name)
     # Filter out warnings caused by local configuration.
     string(REGEX REPLACE "[^\n]*warning:[^\n]*directory not found for option[^\n]*" "" check_output "${check_output}")
     string(REGEX REPLACE "[^\n]*warning:[^\n]*object file compiled with -mlong-branch which is no longer needed[^\n]*" "" check_output "${check_output}")
+    # Filter out other warnings unrelated to feature checks.
+    string(REGEX REPLACE "[^\n]*warning:[^\n]*sprintf\\(\\) is often misused, please use snprintf[^\n]*" "" check_output "${check_output}")
     # Filter out xcodebuild warnings.
     string(REGEX REPLACE "[^\n]* xcodebuild\\[[0-9]*:[0-9]*\\] warning: [^\n]*" "" check_output "${check_output}")
     # If using the feature causes warnings, treat it as broken/unavailable.