Add POST_BUILD_COPY_EXT_LIBS option in CMakeLists.txt

[joni999]

Hi!
I'm in the process of creating Arch Linux AUR packages of all the libraries necessary to build & use vpinball and noticed that I can't turn off the copy of the external libraries in this project.
I don't use your external.sh build script, because I installed libbass.so right into my system with the existing AUR package and want to link it dynamically.

This is my first pull request - I hope it's okay :)
Any feedback is welcome!

Before:

(base) [joni@linuxjoni04 libaltsound]$ cmake -DPLATFORM=linux -DARCH=x64 -DCMAKE_BUILD_TYPE=Release -DBUILD_STATIC=FALSE -DCMAKE_BUILD_WITH_INSTALL_RPATH=FALSE  -B build
-- PLATFORM: linux
-- ARCH: x64
-- BUILD_SHARED: ON
-- BUILD_STATIC: FALSE
-- POST_BUILD_COPY_EXT_LIBS: ON
-- The C compiler identification is GNU 15.1.1
-- The CXX compiler identification is GNU 15.1.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.3s)
-- Generating done (0.0s)
-- Build files have been written to: /home/joni/git/libaltsound/build
(base) [joni@linuxjoni04 libaltsound]$ cmake --build build -- -j$(nproc)
[  7%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_data.cpp.o
[ 15%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_file_parser.cpp.o
[ 23%] Building CXX object CMakeFiles/altsound_shared.dir/src/gsound_processor.cpp.o
[ 30%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound.cpp.o
[ 38%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_logger.cpp.o
[ 46%] Building CXX object CMakeFiles/altsound_shared.dir/src/gsound_csv_parser.cpp.o
[ 53%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_processor.cpp.o
[ 61%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_ini_processor.cpp.o
[ 69%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_processor_base.cpp.o
[ 76%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_csv_parser.cpp.o
[ 84%] Linking CXX shared library libaltsound.so
[ 84%] Built target altsound_shared
[ 92%] Building CXX object CMakeFiles/altsound_test.dir/src/test.cpp.o
[100%] Linking CXX executable altsound_test
Error copying file "/home/joni/git/libaltsound/third-party/runtime-libs/linux/x64/libbass.so" to "/home/joni/git/libaltsound/build".
make[2]: *** [CMakeFiles/altsound_test.dir/build.make:103: altsound_test] Error 1
make[2]: *** Deleting file 'altsound_test'
make[1]: *** [CMakeFiles/Makefile2:122: CMakeFiles/altsound_test.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

After:

(base) [joni@linuxjoni04 libaltsound]$ cmake -DPLATFORM=linux -DARCH=x64 -DCMAKE_BUILD_TYPE=Release -DBUILD_STATIC=FALSE -DCMAKE_BUILD_WITH_INSTALL_RPATH=FALSE -DPOST_BUILD_COPY_EXT_LIBS=FALSE -B build
-- PLATFORM: linux
-- ARCH: x64
-- BUILD_SHARED: ON
-- BUILD_STATIC: FALSE
-- POST_BUILD_COPY_EXT_LIBS: FALSE
-- The C compiler identification is GNU 15.1.1
-- The CXX compiler identification is GNU 15.1.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: /home/joni/git/libaltsound/build
(base) [joni@linuxjoni04 libaltsound]$ cmake --build build -- -j$(nproc)
[  7%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_logger.cpp.o
[ 23%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_file_parser.cpp.o
[ 23%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_csv_parser.cpp.o
[ 30%] Building CXX object CMakeFiles/altsound_shared.dir/src/gsound_processor.cpp.o
[ 38%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound.cpp.o
[ 53%] Building CXX object CMakeFiles/altsound_shared.dir/src/gsound_csv_parser.cpp.o
[ 53%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_data.cpp.o
[ 61%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_ini_processor.cpp.o
[ 69%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_processor_base.cpp.o
[ 76%] Building CXX object CMakeFiles/altsound_shared.dir/src/altsound_processor.cpp.o
[ 84%] Linking CXX shared library libaltsound.so
[ 84%] Built target altsound_shared
[ 92%] Building CXX object CMakeFiles/altsound_test.dir/src/test.cpp.o
[100%] Linking CXX executable altsound_test
[100%] Built target altsound_test