CMake

Add --enable-opus-custom-api for QEXT #711

Job	Run time
CMake MINGW	1m 4s
CMake/Android/So/ARMv8/Release	39s
CMake/Android/So/X86/Release	29s
Test build with CMake 3.16.0	1m 7s
CMake/Android/So/X64/Release	24s
CMake/Windows/Dll/X86/Release	1m 35s
CMake/Windows/Dll/X64/Release	1m 38s
CMake/Windows/Dll/ARMv8/Release	1m 42s
CMake/Linux/So/X64/Release	30s
CMake/MacOSX/So/X64/Release	30s
CMake/MacOSX/Framework/X64/Release	27s
CMake/iOS/Dll/arm64/Release	51s
CMake/iOS/Framework/arm64/Release	29s
CMake/Windows/Lib/X86/Release	1m 8s
CMake/Windows/Lib/X64/Release	1m 38s
CMake/Windows/Lib/armv8/Release	1m 32s
CMake/Linux/Lib/X64/Release	31s
CMake/MacOSX/Lib/X64/Release	29s
CMake/iOS/Lib/arm64/Release	31s
CMake/Android/Lib/ARMv8/Release	55s
CMake/Android/Lib/X86/Release	34s
CMake/Android/Lib/X64/Release	39s
CMake/CustomModes/Linux/Lib/X64/Release	30s
CMake/AssertionsFuzz/Windows/Lib/X64/Release	1m 20s
CMake/AssertionsFuzz/Linux/Lib/X64/Release	29s
CMake/AssertionsFuzz/MacOSX/Lib/X64/Release	35s
	22m 16s

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