Missing `EXCLUDE_FROM_ALL` for Oboe from source

First of all, thanks for developing and maintaining OpenAL Soft. It is much appreciated.

While making prebuilt binaries of OpenAL Soft for Android with Oboe support for our project, I noticed that Oboe headers and static library archives get installed alongside OpenAL Soft itself when running cmake --install (or equivalent).

The resaon seems to be that the Oboe CMakeLists file uses the following install commands:
https://github.com/google/oboe/blob/03242e9ef9495e418e6ae83954d239dc9193ec5c/CMakeLists.txt#L100-L106

Those get inherited by the OpenAL Soft library target because add_directory() is not using EXCLUDE_FROM_ALL here:

openal-soft/CMakeLists.txt

Lines 1111 to 1114 in d3875f3

    
           if(OBOE_SOURCE) 
        
               add_subdirectory(${OBOE_SOURCE} ./oboe) 
        
               set(OBOE_TARGET oboe) 
        
           else()

After building and installing OpenAL Soft, the following is the resulting file-tree in CMAKE_INSTALL_PREFIX:

include/oboe/StabilizedCallback.h
include/oboe/FullDuplexStream.h
include/oboe/Utilities.h
include/oboe/ResultWithValue.h
include/oboe/Definitions.h
include/oboe/OboeExtensions.h
include/oboe/FifoControllerBase.h
include/oboe/Oboe.h
include/oboe/AudioStreamBase.h
include/oboe/LatencyTuner.h
include/oboe/AudioStreamBuilder.h
include/oboe/FifoBuffer.h
include/oboe/AudioStream.h
include/oboe/AudioStreamCallback.h
include/oboe/Version.h
include/AL/al.h
include/AL/efx-creative.h
include/AL/alext.h
include/AL/alc.h
include/AL/efx.h
include/AL/efx-presets.h
lib/pkgconfig/openal.pc
lib/cmake/OpenAL/OpenALTargets-relwithdebinfo.cmake
lib/cmake/OpenAL/OpenALTargets.cmake
lib/cmake/OpenAL/OpenALConfig.cmake
lib/libopenal.so
lib/x86_64/liboboe.a

When I add EXCLUDE_FROM_ALL to the Oboe add_directory() command, the resulting file-tree only contains OpenAL Soft headers/libs as expected:

include/AL/al.h
include/AL/efx-creative.h
include/AL/alext.h
include/AL/alc.h
include/AL/efx.h
include/AL/efx-presets.h
lib/pkgconfig/openal.pc
lib/cmake/OpenAL/OpenALTargets-relwithdebinfo.cmake
lib/cmake/OpenAL/OpenALTargets.cmake
lib/cmake/OpenAL/OpenALConfig.cmake
lib/libopenal.so

I understand that Oboe from source is compiled statically into libopenal.so, and that OpenAL Soft abstracts Oboe as a backend. Therefore there should be no need to install Oboe header/lib files alongside OpenAL Soft.

Should Oboe from source be added using EXCLUDE_FROM_ALL?
I can make a quick PR fixing this if you agree. Cheers!

It would seem to make sense to add EXCLUDE_FROM_ALL to the add_subdirectory for Oboe, yeah. I wasn't aware it also caused Oboe's headers and libs to get installed alongside OpenAL Soft's.

	if(OBOE_SOURCE)
	add_subdirectory(${OBOE_SOURCE} ./oboe)
	set(OBOE_TARGET oboe)
	else()