find*: Add options to ignore entire prefixes

I'm trying to prevent find_library and find_package from considering /usr/local as a system prefix path on macOS, because that location usually contains libraries installed by Homebrew. I want to limit it to the SDK locations only.

I can achieve that for find_library by setting either CMAKE_IGNORE_PATH or CMAKE_SYSTEM_IGNORE_PATH. But that doesn't work for find_package (and the docs for the variables don't mention find_package).

Is there a reason for that?

Setting NO_CMAKE_SYSTEM_PATH for find_package is a no-go as well, because that disables a lot more paths.

It's also a bit inconsistent that the docs for CMAKE_SYSTEM_PREFIX_PATH mention CMAKE_SYSTEM_IGNORE_PATH, but the latter doesn't affect find_package even though the former does.

Here's a sample project to demonstrate, i have harfbuzz installed in /usr/local:

cmake_minimum_required(VERSION 3.15.0)

set(CMAKE_SYSTEM_IGNORE_PATH "/usr/local/lib;/usr/local/lib/cmake;/usr/X11R6/lib;/opt;/usr/pkg")
project(app LANGUAGES CXX)

set(CMAKE_IGNORE_PATH "/usr/local/lib;/usr/local/lib/cmake;/usr/X11R6/lib;/opt;/usr/pkg")
set(CMAKE_FIND_DEBUG_MODE TRUE)
find_package(harfbuzz
    NO_SYSTEM_ENVIRONMENT_PATH)
unset(CMAKE_FIND_DEBUG_MODE)

$ cmake ..

-- The CXX compiler identification is AppleClang 11.0.0.11000033
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++
-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Debug Log at CMakeLists.txt:8 (find_package):
  <PackageName>_ROOT CMake variable [CMAKE_FIND_USE_PACKAGE_ROOT_PATH].

    none

  CMAKE_PREFIX_PATH variable [CMAKE_FIND_USE_CMAKE_PATH].

    none

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH variables
  [CMAKE_FIND_USE_CMAKE_PATH].

    none

  Env variable harfbuzz_DIR [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_PREFIX_PATH env variable [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH env variables
  [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  Paths specified by the find_package HINTS option.

    none

  CMake User Package Registry [CMAKE_FIND_USE_PACKAGE_REGISTRY].

    none

  CMake variables defined in the Platform file
  [CMAKE_FIND_USE_CMAKE_SYSTEM_PATH].

    /usr/local
    /usr
    /
    /Users/alex
    /usr/X11R6
    /usr/pkg
    /opt
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr
    /sw
    /opt/local
    /Users/alex/Library/Frameworks
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/Library/Frameworks
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/Network/Library/Frameworks
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks
    /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks
    /Applications/Xcode.app/Contents/Developer/Library/Frameworks
    /Library/Frameworks
    /Network/Library/Frameworks
    /System/Library/Frameworks
    /Users/alex/Applications
    /Applications
    /Applications/Xcode.app/Contents/Applications
    /Applications/Xcode.app/Contents/Developer/Applications



  CMake System Package Registry
  [CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY].

    none

  Paths specified by the find_package PATHS option.

    none

  find_package considered the following locations for the Config module:

    /usr/local/harfbuzzConfig.cmake
    /usr/local/harfbuzz-config.cmake
    /usr/local/lib/cmake/harfbuzz/harfbuzzConfig.cmake
    /usr/local/lib/cmake/harfbuzz/harfbuzz-config.cmake

  The file was found at

    /usr/local/lib/cmake/harfbuzz/harfbuzz-config.cmake



-- Configuring done
-- Generating done