Android: Populate CMAKE_HOST_SYSTEM_VERSION on native builds

Follow up commit c5892e484e (Android: add support for native
compilation, such as with the Termux app, 2019-10-15,
v3.17.0-rc1~594^2).

Modules/CMakeDetermineSystem.cmake

@@ -47,6 +47,28 @@ if(CMAKE_HOST_UNIX)
       set(CMAKE_HOST_SYSTEM_VERSION "${_CMAKE_HOST_SYSTEM_MAJOR_VERSION}.${_CMAKE_HOST_SYSTEM_MINOR_VERSION}")
       unset(_CMAKE_HOST_SYSTEM_MAJOR_VERSION)
       unset(_CMAKE_HOST_SYSTEM_MINOR_VERSION)
+    elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+      execute_process(COMMAND getprop ro.build.version.sdk
+        OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+        ERROR_QUIET)
+
+      if(NOT DEFINED CMAKE_SYSTEM_VERSION)
+        set(_ANDROID_API_LEVEL_H $ENV{PREFIX}/include/android/api-level.h)
+        set(_ANDROID_API_REGEX "#define __ANDROID_API__ ([0-9]+)")
+        file(READ ${_ANDROID_API_LEVEL_H} _ANDROID_API_LEVEL_H_CONTENT)
+        string(REGEX MATCH ${_ANDROID_API_REGEX} _ANDROID_API_LINE "${_ANDROID_API_LEVEL_H_CONTENT}")
+        string(REGEX REPLACE ${_ANDROID_API_REGEX} "\\1" _ANDROID_API "${_ANDROID_API_LINE}")
+        if(_ANDROID_API)
+          set(CMAKE_SYSTEM_VERSION "${_ANDROID_API}")
+        endif()
+
+        unset(_ANDROID_API_LEVEL_H)
+        unset(_ANDROID_API_LEVEL_H_CONTENT)
+        unset(_ANDROID_API_REGEX)
+        unset(_ANDROID_API_LINE)
+        unset(_ANDROID_API)
+      endif()
     else()
       execute_process(COMMAND ${CMAKE_UNAME} -r
         OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION