Android: restructure android search paths

1. Set CMAKE_FIND_ROOT_PATH unconditionally.

Revise the implementation from commit a7f41a7ee4 (Android: Fix find_*
search order within NDK for unified toolchains, 2020-10-13).  In the old
implementation, if people set CMAKE_FIND_ROOT_PATH, CMAKE_ANDROID_NDK
won't be added to find root. And all paths added to CMAKE_SYSTEM_*_PATH
below will be rerooted to the user specified root.

2. Add api level specific library path to CMAKE_SYSTEM_PREFIX_PATH.

As the discussion in [1], some people want the paths added by
UnixPaths.cmake. They install their libraries according to
GNUInstallDirs [2].

As a result, we cannot clear CMAKE_SYSTEM_PREFIX_PATH. It includes /usr
so no matter what we specify in CMAKE_SYSTEM_LIBRARY_PATH,
/usr/lib/<arch> will be searched first.

The author also pointed out a way to solve this issue [3]. In addition
to other paths, CMake also searches <root>/<prefix> [4]. So we can add
the API specific lib path to the beginning of CMAKE_SYSTEM_PREFIX_PATH,
to have it searched first.

[1] https://android-review.googlesource.com/c/platform/ndk/+/1486800
[2] https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
[3] https://github.com/android/ndk/issues/1179#issuecomment-613435081
[4] https://gitlab.kitware.com/cmake/cmake/-/blob/11425041f04fd0945480b8f9e9933d1549b93981/Source/cmSearchPath.cxx#L202

2 files changed, 24 insertions, 0 deletions

diff --git a/Tests/RunCMake/Android/RunCMakeTest.cmake b/Tests/RunCMake/Android/RunCMakeTest.cmake
index c4b1a00..aa0cf4d 100644
--- a/Tests/RunCMake/Android/RunCMakeTest.cmake
+++ b/Tests/RunCMake/Android/RunCMakeTest.cmake
@@ -103,6 +103,13 @@ foreach(ndk IN LISTS TEST_ANDROID_NDK)
     set(ndk_arg)
   endif()
 
+  set(RunCMake_TEST_OPTIONS
+    -DCMAKE_SYSTEM_NAME=Android
+    -DCMAKE_FIND_ROOT_PATH=/tmp
+    ${ndk_arg}
+    )
+  run_cmake(ndk-search-order)
+
   # Test failure cases.
   message(STATUS "ndk='${ndk}'")
   if(RunCMake_GENERATOR MATCHES "Visual Studio")
diff --git a/Tests/RunCMake/Android/ndk-search-order.cmake b/Tests/RunCMake/Android/ndk-search-order.cmake
new file mode 100644
index 0000000..498d775
--- /dev/null
+++ b/Tests/RunCMake/Android/ndk-search-order.cmake
@@ -0,0 +1,17 @@
+if(NOT CMAKE_ANDROID_NDK_TOOLCHAIN_UNIFIED)
+  return()
+endif()
+
+find_library(LIBDL dl)
+if(NOT LIBDL)
+  message(FATAL_ERROR "libdl not found.")
+endif()
+
+if(LIBDL MATCHES ".a$")
+  message(FATAL_ERROR "found libdl.a")
+endif()
+
+find_program(CLANG clang)
+if(NOT CLANG)
+  message(FATAL_ERROR "clang not found")
+endif()