Do not prepend SDK path to CMAKE_FIND_ROOT_PATH

[mologie]

Prepending to CMAKE_FIND_ROOT_PATH is harmful [1], because if a library exists in both the SDK and in the user's path (via e.g. the Conan Package Manager), the SDK's library is used. Linking against the wrong library (e.g. libprotobuf on iOS) may crash the app or get it rejected from the store.

Compared to the NDK issue I references we can just drop modification of CMAKE_FIND_ROOT_PATH completely because CMake searches in the custom CMAKE_OSX_SYSROOT anyway.

This pull request implements the suggested change and fixes find_host_package to properly restore the user's find root settings.

[1] android/ndk#912

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Here is a test case to confirm that libraries available in the iOS SDK are still found after this patch:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.0.0 FATAL_ERROR)
project(test C)
find_package(ZLIB)

$ cmake -S . -B build -D CMAKE_TOOLCHAIN_FILE=~/devel/ios-cmake/ios.toolchain.cmake -D PLATFORM=OS64 
-- [DEFAULTS] Using the default min-version since DEPLOYMENT_TARGET not provided!
-- [DEFAULTS] Enabling bitcode support by default. ENABLE_BITCODE not provided!
-- [DEFAULTS] Enabling ARC support by default. ENABLE_ARC not provided!
-- [DEFAULTS] Hiding symbols visibility by default. ENABLE_VISIBILITY not provided!
-- [DEFAULTS] Using NON-strict compiler checks by default. ENABLE_STRICT_TRY_COMPILE not provided!
-- Configuring iphoneos build for platform: OS64, architecture(s): arm64
-- Using SDK: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.4.sdk
-- Using C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
-- Using CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++
-- Using libtool: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool
-- Using install name tool: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/install_name_tool
-- Autoconf target triple: aarch64-apple-ios
-- Using minimum deployment version: 11.0 (SDK version: 14.4)
-- Merging integrated CMake 3.14+ iOS,tvOS,watchOS,macOS toolchain(s) with this toolchain!
-- CMake version: 3.19.6
-- Using a data_ptr size of: 8
-- Bitcode: Enabled
-- ARC: Enabled
-- Hiding symbols: Enabled
-- The C compiler identification is AppleClang 12.0.0.12000032
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Found ZLIB: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.4.sdk/usr/lib/libz.tbd (found version "1.2.11") 
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/ios-cmake-find-bug/build

[leetal]

This change seems to have broken the tests somehow? Perhaps a better idea would be to append to any existing CMAKE_FIND_ROOT_PATH (as the Android toolchain does)?

[mologie]

I don't think prepending is a solution, it will still break package managers / Conan that way. This seems to be an issue with how the toolchain behaves when targetting Xcode -- I will look into it! Please just keep this PR open meanwhile.

[leetal]

I have made some changes to this now on master. I cannot verify if it works or not since i personally don't use Conan, but i think that if you specify CMAKE_FIND_ROOT_PATH to the Conan directories the changes will allow the find_* calls to actually lookup the Conan versions instead of the toolchain versions.