In May 2017 the library loading was limited to *.dylib but the macOS
system OpenAL framework does not have dylib extension. So allow loading
files from /System/Library/Frameworks/ as libraries even without dylib
extension.
This is checked in Sys_DllExtension() so that QVM filesystem access will
not allow writing files to /System/Library/Frameworks/ even if homepath
is changed to include it. (Admittedly it doesn't fit the function name
but fits the function description and current usage.)
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