[cmake] Run normal compiler auto-detection by default

This change disables `EMSCRIPTEN_FORCE_COMPILERS` by default which means
cmake will run all the normal compiler detection phases.  This has the
advantage that things get setup correctly without taking any shortcuts.
See emscripten-core#23444.

The downside is the that cmake phase itself is now slower since its
needs to compiler, link and run a bunch of test binaries.

On my machine running all the cmake tests went from 27 to 36 seconds.
Running a single empty cmake project when from 1s to 4.5s.  Since
running cmake is normally rare compared to building the project itself I
think this slowdown is worth it, but I've left the
`EMSCRIPTEN_FORCE_COMPILERS` setting in place for now that folks that
want fast cmake.

Also, remember that cmake itself caches all this stuff so its only the
first run that get slowed down.

ChangeLog.md

@@ -31,6 +31,10 @@ See docs/process.md for more on how version tagging works.
 - The system JS libraries in `src/` were renamed from `library_foo.js` to
   `lib/libfoo.js`. They are still included via the same `-lfoo.js` flag so
   this should not be a user-visible change. (#23348)
+- When using cmake the emscripten toolchain will no longer skip the toolchain
+  detection stages.  This means the cmake run slower but will result in more
+  accruate information.  If cmake is running too slow for you, you can rever to
+  the previous behaviour with `-DEMSCRIPTEN_FORCE_COMPILERS=ON`.
 
 4.0.1 - 01/17/25
 ----------------

cmake/Modules/Platform/Emscripten.cmake

@@ -120,11 +120,9 @@ endif()
 file(TO_CMAKE_PATH "${_emcache_output}" _emcache_output)
 set(EMSCRIPTEN_SYSROOT "${_emcache_output}/sysroot")
 
-# Don't allow CMake to autodetect the compiler, since this is quite slow with
-# Emscripten.
-# Pass -DEMSCRIPTEN_FORCE_COMPILERS=OFF to disable (sensible mostly only for
-# testing/debugging purposes).
-option(EMSCRIPTEN_FORCE_COMPILERS "Force C/C++ compiler" ON)
+# Allow skipping of CMake compiler autodetection, since this is quite slow with
+# Emscripten. Pass -DEMSCRIPTEN_FORCE_COMPILERS=ON to enable
+option(EMSCRIPTEN_FORCE_COMPILERS "Force C/C++ compiler" OFF)
 if (EMSCRIPTEN_FORCE_COMPILERS)
 
   # Detect version of the 'emcc' executable. Note that for CMake, we tell it the

test/test_other.py

@@ -966,7 +966,11 @@ def test_cmake(self, test_dir, output_file, cmake_args):
   # If we update LLVM version and this test fails, copy over the new advertised features from Clang
   # and place them to cmake/Modules/Platform/Emscripten.cmake.
   @no_windows('Skipped on Windows because CMake does not configure native Clang builds well on Windows.')
-  def test_cmake_compile_features(self):
+  @parameterized({
+    '': ([],),
+    'force': (['-DEMSCRIPTEN_FORCE_COMPILERS=ON'],),
+  })
+  def test_cmake_compile_features(self, args):
     os.mkdir('build_native')
     cmd = ['cmake',
            '-DCMAKE_C_COMPILER=' + CLANG_CC, '-DCMAKE_C_FLAGS=--target=' + clang_native.get_native_triple(),
@@ -976,7 +980,7 @@ def test_cmake_compile_features(self):
     native_features = self.run_process(cmd, stdout=PIPE, cwd='build_native').stdout
 
     os.mkdir('build_emcc')
-    cmd = [EMCMAKE, 'cmake', test_file('cmake/stdproperty')]
+    cmd = [EMCMAKE, 'cmake', test_file('cmake/stdproperty')] + args
     print(str(cmd))
     emscripten_features = self.run_process(cmd, stdout=PIPE, cwd='build_emcc').stdout
 
@@ -1023,7 +1027,7 @@ def test_cmake_bitcode_static_libraries(self):
   @crossplatform
   @parameterized({
     '': ([],),
-    'noforce': (['-DEMSCRIPTEN_FORCE_COMPILERS=OFF'],),
+    'force': (['-DEMSCRIPTEN_FORCE_COMPILERS=ON'],),
   })
   def test_cmake_compile_commands(self, args):
     self.run_process([EMCMAKE, 'cmake', test_file('cmake/static_lib'), '-DCMAKE_EXPORT_COMPILE_COMMANDS=ON'] + args)