Ninja: On Windows, select the compiler occurring first in PATH

On Windows, toolchains like MSVC require a set of environment variables
to be configured for the compiler to work correctly.  The scripts that
prepare these environments typically put the compiler's directory ahead
of others in the `PATH`.  Teach the Ninja generators to use this as a
heuristic to select the compiler when none is explicitly specified.

This is not necessary with Makefile generators because each toolchain's
environment comes with its own make tool variant, and the corresponding
Makefile generator (e.g. "NMake Makefiles") automatically implies the
matching compiler.

Fixes: #20585

Help/release/dev/ninja-compiler-PATH-windows.rst

@@ -0,0 +1,7 @@
+ninja-compiler-PATH-windows
+---------------------------
+
+* On Windows, the :generator:`Ninja` and :generator:`Ninja Multi-Config`
+  generators, when a compiler is not explicitly specified, now select
+  the first compiler (of any name) found in directories listed by the
+  ``PATH`` environment variable.

Modules/CMakeDetermineCompiler.cmake

@@ -53,6 +53,20 @@ macro(_cmake_find_compiler lang)
       NO_DEFAULT_PATH
       DOC "${lang} compiler")
   endif()
+  if(CMAKE_HOST_WIN32 AND CMAKE_GENERATOR MATCHES "Ninja")
+    # On Windows command-line builds, the Makefile generators each imply
+    # a preferred compiler tool.  The Ninja generator does not imply a
+    # compiler tool, so use the compiler that occurs first in PATH.
+    find_program(CMAKE_${lang}_COMPILER
+      NAMES ${CMAKE_${lang}_COMPILER_LIST}
+      NAMES_PER_DIR
+      DOC "${lang} compiler"
+      NO_PACKAGE_ROOT_PATH
+      NO_CMAKE_PATH
+      NO_CMAKE_ENVIRONMENT_PATH
+      NO_CMAKE_SYSTEM_PATH
+      )
+  endif()
   find_program(CMAKE_${lang}_COMPILER NAMES ${CMAKE_${lang}_COMPILER_LIST} DOC "${lang} compiler")
   if(CMAKE_${lang}_COMPILER_INIT AND NOT CMAKE_${lang}_COMPILER)
     set_property(CACHE CMAKE_${lang}_COMPILER PROPERTY VALUE "${CMAKE_${lang}_COMPILER_INIT}")

Tests/RunCMake/Ninja/RunCMakeTest.cmake

@@ -18,6 +18,12 @@ else()
   message(FATAL_ERROR "'ninja --version' reported:\n${ninja_out}")
 endif()
 
+if(CMAKE_HOST_WIN32)
+  run_cmake(SelectCompilerWindows)
+else()
+  run_cmake(SelectCompilerUNIX)
+endif()
+
 function(run_NinjaToolMissing)
   set(RunCMake_MAKE_PROGRAM ninja-tool-missing)
   run_cmake(NinjaToolMissing)

Tests/RunCMake/Ninja/SelectCompiler/1/gcc

@@ -0,0 +1,2 @@
+#!/bin/sh
+false

Tests/RunCMake/Ninja/SelectCompiler/2/cc

@@ -0,0 +1,2 @@
+#!/bin/sh
+false

Tests/RunCMake/Ninja/SelectCompilerUNIX-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/Ninja/SelectCompilerUNIX-stderr.txt

@@ -0,0 +1,6 @@
+^CMake Error at .*/Modules/CMakeTestCCompiler.cmake:[0-9]+ \(message\):
+  The C compiler
+
+    "[^"]*/Tests/RunCMake/Ninja/SelectCompiler/2/cc"
+
+  is not able to compile a simple test program.

Tests/RunCMake/Ninja/SelectCompilerUNIX.cmake

@@ -0,0 +1,3 @@
+set(ENV{PATH} "${CMAKE_CURRENT_SOURCE_DIR}/SelectCompiler/1:${CMAKE_CURRENT_SOURCE_DIR}/SelectCompiler/2:$ENV{PATH}")
+unset(ENV{CC})
+enable_language(C)

Tests/RunCMake/Ninja/SelectCompilerWindows-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/Ninja/SelectCompilerWindows-stderr.txt

@@ -0,0 +1,6 @@
+^CMake Error at .*/Modules/CMakeTestCCompiler.cmake:[0-9]+ \(message\):
+  The C compiler
+
+    "[^"]*/Tests/RunCMake/Ninja/SelectCompiler/1/gcc"
+
+  is not able to compile a simple test program.

Tests/RunCMake/Ninja/SelectCompilerWindows.cmake

@@ -0,0 +1,3 @@
+set(ENV{PATH} "${CMAKE_CURRENT_SOURCE_DIR}/SelectCompiler/1;${CMAKE_CURRENT_SOURCE_DIR}/SelectCompiler/2;$ENV{PATH}")
+unset(ENV{CC})
+enable_language(C)