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

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

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:

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
 

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}")

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)

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()

Source/cmStateSnapshot.cxx

@@ -315,10 +315,14 @@ void cmStateSnapshot::SetDefaultDefinitions()
   this->SetDefinition("UNIX", "1");
   this->SetDefinition("CMAKE_HOST_UNIX", "1");
 
+#  if defined(__ANDROID__)
+  this->SetDefinition("CMAKE_HOST_SYSTEM_NAME", "Android");
+#  else
   struct utsname uts_name;
   if (uname(&uts_name) >= 0) {
     this->SetDefinition("CMAKE_HOST_SYSTEM_NAME", uts_name.sysname);
   }
+#  endif
 #endif
 #if defined(__CYGWIN__)
   std::string legacy;