Fix Emscripten detection false positives and unsuppressible status messages

Issue 1: False positive Emscripten detection

The check_c_source_compiles() test incorrectly detected Emscripten when
building with non-Emscripten WASM toolchains (e.g., WASI SDK). In cross-
compilation scenarios, CMake creates a static library instead of an
executable for compile tests. An empty source file (when __EMSCRIPTEN__
is not defined) successfully archives into a static library, causing the
test to pass when it should fail.

Resolution: Use the EMSCRIPTEN CMake variable set by Emscripten's official
toolchain file, which is the recommended detection method. Remove the
unreliable check_c_source_compiles() fallback.

Issue 2: Unsuppressible status messages

The message() calls lacked a type argument, making them always print
regardless of CMake's log level settings.

Resolution: Add STATUS type to message() calls so they can be suppressed
with CMake's --log-level option.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d27ecf8..721328e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,17 +17,23 @@
   message(STATUS "Build type is '${CMAKE_BUILD_TYPE}'")
 endif()
 
-include(CheckCSourceCompiles)
-check_c_source_compiles(
-  "#if defined(__EMSCRIPTEN__)
-   int main() {return 0;}
-   #endif"
-  BROTLI_EMSCRIPTEN
-)
-if (BROTLI_EMSCRIPTEN)
-  message("-- Compiler is EMSCRIPTEN")
+# Detect Emscripten compiler.
+# The EMSCRIPTEN variable is set by Emscripten's CMake toolchain file and is
+# the recommended way to detect Emscripten builds.
+#
+# Note: Previously this used check_c_source_compiles() to test for the
+# __EMSCRIPTEN__ macro, but that approach is unreliable in cross-compilation
+# scenarios where CMake links a static library instead of an executable,
+# causing false positives for non-Emscripten WASM toolchains (e.g., WASI SDK).
+if (EMSCRIPTEN)
+  set(BROTLI_EMSCRIPTEN TRUE)
 else()
-  message("-- Compiler is not EMSCRIPTEN")
+  set(BROTLI_EMSCRIPTEN FALSE)
+endif()
+if (BROTLI_EMSCRIPTEN)
+  message(STATUS "Compiler is EMSCRIPTEN")
+else()
+  message(STATUS "Compiler is not EMSCRIPTEN")
 endif()
 
 if (BROTLI_EMSCRIPTEN)