gstrfuncs: Fix regression in C++ types accepted by g_str_equal()

Philip Withnallrequested to merge
pwithnall/glib:2820-str-equal-cxx into main

Further to commit bcd364af, fix the types accepted by the g_str_equal() macro for C++ too. C++ is more restrictive about const-correctness.

Add unit tests.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #2820 (closed)

Closes #2820 (closed)

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