Android: Avoid interfering with common pre-existing toolchain files

Commonly used Android toolchain files that pre-date CMake upstream
support may need to be updated to work with our new functionality.
They typically set CMAKE_SYSTEM_VERSION to 1, so detect that and
skip our upstream Android settings.  When such toolchain files are
updated to account for our upstream support, they can then set
CMAKE_SYSTEM_VERSION to a valid Android API and get new behavior.

Modules/Platform/Android-Clang.cmake

@@ -26,6 +26,14 @@ if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
   return()
 endif()
 
+# Commonly used Android toolchain files that pre-date CMake upstream support
+# set CMAKE_SYSTEM_VERSION to 1.  Avoid interfering with them.
+if(CMAKE_SYSTEM_VERSION EQUAL 1)
+  macro(__android_compiler_clang lang)
+  endmacro()
+  return()
+endif()
+
 include(Platform/Android-Common)
 
 macro(__android_compiler_clang lang)

Modules/Platform/Android-Determine.cmake

@@ -22,6 +22,12 @@ if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
   return()
 endif()
 
+# Commonly used Android toolchain files that pre-date CMake upstream support
+# set CMAKE_SYSTEM_VERSION to 1.  Avoid interfering with them.
+if(CMAKE_SYSTEM_VERSION EQUAL 1)
+  return()
+endif()
+
 # If the user provided CMAKE_SYSROOT for us, extract information from it.
 set(_ANDROID_SYSROOT_NDK "")
 set(_ANDROID_SYSROOT_API "")

Modules/Platform/Android-GNU.cmake

@@ -26,6 +26,14 @@ if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
   return()
 endif()
 
+# Commonly used Android toolchain files that pre-date CMake upstream support
+# set CMAKE_SYSTEM_VERSION to 1.  Avoid interfering with them.
+if(CMAKE_SYSTEM_VERSION EQUAL 1)
+  macro(__android_compiler_gnu lang)
+  endmacro()
+  return()
+endif()
+
 include(Platform/Android-Common)
 
 macro(__android_compiler_gnu lang)

Modules/Platform/Android-Initialize.cmake

@@ -21,6 +21,12 @@ if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
   return()
 endif()
 
+# Commonly used Android toolchain files that pre-date CMake upstream support
+# set CMAKE_SYSTEM_VERSION to 1.  Avoid interfering with them.
+if(CMAKE_SYSTEM_VERSION EQUAL 1)
+  return()
+endif()
+
 if(NOT CMAKE_SYSROOT)
   if(CMAKE_ANDROID_NDK)
     set(CMAKE_SYSROOT "${CMAKE_ANDROID_NDK}/platforms/android-${CMAKE_SYSTEM_VERSION}/arch-${CMAKE_ANDROID_ARCH}")

Modules/Platform/Android/Determine-Compiler.cmake

@@ -26,6 +26,14 @@ if(CMAKE_VS_PLATFORM_NAME STREQUAL "Tegra-Android")
   return()
 endif()
 
+# Commonly used Android toolchain files that pre-date CMake upstream support
+# set CMAKE_SYSTEM_VERSION to 1.  Avoid interfering with them.
+if(CMAKE_SYSTEM_VERSION EQUAL 1)
+  macro(__android_determine_compiler lang)
+  endmacro()
+  return()
+endif()
+
 # Identify the host platform.
 if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
   set(_ANDROID_HOST_EXT "")