Android: add support for native compilation, such as with the Termux app

The CMake support for Android assumes cross-compilation using the NDK,
so stub out that Android NDK support and use the Linux support that's
already invoked. Set CMAKE_HOST_SYSTEM_NAME to "Android", rather than
"Linux".

Issue: #19840

6 files changed, 36 insertions, 1 deletions

diff --git a/Modules/CMakeDetermineSystem.cmake b/Modules/CMakeDetermineSystem.cmake
index dc208c6..f3ec4da 100644
--- a/Modules/CMakeDetermineSystem.cmake
+++ b/Modules/CMakeDetermineSystem.cmake
@@ -43,7 +43,7 @@ if(CMAKE_HOST_UNIX)
     else()
       exec_program(${CMAKE_UNAME} ARGS -r OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION)
     endif()
-    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin|^GNU$")
+    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin|^GNU$|Android")
       exec_program(${CMAKE_UNAME} ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)
       if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin" AND
diff --git a/Modules/Platform/Android-Clang.cmake b/Modules/Platform/Android-Clang.cmake
index 847178f..759448b 100644
--- a/Modules/Platform/Android-Clang.cmake
+++ b/Modules/Platform/Android-Clang.cmake
@@ -24,6 +24,14 @@ if(CMAKE_SYSTEM_VERSION EQUAL 1)
   return()
 endif()
 
+# Natively compiling on an Android host doesn't use the NDK cross-compilation
+# tools.
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+  macro(__android_compiler_clang lang)
+  endmacro()
+  return()
+endif()
+
 include(Platform/Android-Common)
 
 # The NDK toolchain configuration files at:
diff --git a/Modules/Platform/Android-Determine.cmake b/Modules/Platform/Android-Determine.cmake
index e7c1b48..2225897 100644
--- a/Modules/Platform/Android-Determine.cmake
+++ b/Modules/Platform/Android-Determine.cmake
@@ -18,6 +18,12 @@ if(CMAKE_SYSTEM_VERSION EQUAL 1)
   return()
 endif()
 
+# Natively compiling on an Android host doesn't use the NDK cross-compilation
+# tools.
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+  return()
+endif()
+
 cmake_policy(PUSH)
 cmake_policy(SET CMP0057 NEW) # if IN_LIST
 
diff --git a/Modules/Platform/Android-Initialize.cmake b/Modules/Platform/Android-Initialize.cmake
index a5d2820..b90dd7a 100644
--- a/Modules/Platform/Android-Initialize.cmake
+++ b/Modules/Platform/Android-Initialize.cmake
@@ -24,6 +24,12 @@ if(CMAKE_ANDROID_NDK_TOOLCHAIN_UNIFIED)
   return()
 endif()
 
+# Natively compiling on an Android host doesn't use the NDK cross-compilation
+# tools.
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+  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}")
diff --git a/Modules/Platform/Android.cmake b/Modules/Platform/Android.cmake
index f08f841..8ffa1b2 100644
--- a/Modules/Platform/Android.cmake
+++ b/Modules/Platform/Android.cmake
@@ -2,6 +2,11 @@ include(Platform/Linux)
 
 set(ANDROID 1)
 
+# Natively compiling on an Android host doesn't need these flags to be reset.
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+  return()
+endif()
+
 # Conventionally Android does not use versioned soname
 # But in modern versions it is acceptable
 if(NOT DEFINED CMAKE_PLATFORM_NO_VERSIONED_SONAME)
diff --git a/Modules/Platform/Android/Determine-Compiler.cmake b/Modules/Platform/Android/Determine-Compiler.cmake
index 5c6b97b..f9c2d89 100644
--- a/Modules/Platform/Android/Determine-Compiler.cmake
+++ b/Modules/Platform/Android/Determine-Compiler.cmake
@@ -31,6 +31,16 @@ elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux")
   set(_ANDROID_HOST_EXT "")
 elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
   set(_ANDROID_HOST_EXT ".exe")
+elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Android")
+  # Natively compiling on an Android host doesn't use the NDK cross-compilation
+  # tools.
+  macro(__android_determine_compiler lang)
+    # Do nothing
+  endmacro()
+  if(NOT CMAKE_CXX_COMPILER_NAMES)
+    set(CMAKE_CXX_COMPILER_NAMES c++)
+  endif()
+  return()
 else()
   message(FATAL_ERROR "Android: Builds hosted on '${CMAKE_HOST_SYSTEM_NAME}' not supported.")
 endif()