Fix variable name in build script

This merge request fixes the $ARCH variable name in the build script.

Successfully running ./compileAndBuildVLCKit.sh -va x86_64 requires that a build directory be created in the root of the project to avoid the following error:

[info] Finished compiling libvlc for x86_64 with SDK version 13.2, platform iphonesimulator
[info] building universal static libs for iphone
[info] all done
[info] Building static MobileVLCKit.framework
./compileAndBuildVLCKit.sh: line 77: pushd: build: No such file or directory

On Big Sur there was an additional "Boost_INCLUDE_DIR-NOTFOUND" error which required replacing ${Boost_INCLUDE_DIR} with /usr/local/Cellar/boost/1.73.0/include in libvlc/vlc/contrib/contrib-x86_64-apple-iOS-Simulator_9.0/taglib/taglib/CMakeLists.txt.

Merged manually. Thanks!

closed