simplify log2 check; currently we rely more on compiler than build sy…

…stem

PiperOrigin-RevId: 598794971

CMakeLists.txt

@@ -78,27 +78,12 @@ if (ENABLE_SANITIZER)
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fsanitize=${ENABLE_SANITIZER}")
 endif ()
 
-include(CheckFunctionExists)
+include(CheckLibraryExists)
 set(LIBM_LIBRARY)
-CHECK_FUNCTION_EXISTS(log2 LOG2_RES)
-if(NOT LOG2_RES)
-  set(_ORIG_REQ_LIBS "${CMAKE_REQUIRED_LIBRARIES}")
-  set(CMAKE_REQUIRED_LIBRARIES "${CMAKE_REQUIRED_LIBRARIES};m")
-  CHECK_FUNCTION_EXISTS(log2 LOG2_LIBM_RES)
-  if(LOG2_LIBM_RES)
-    set(LIBM_LIBRARY "m")
-    add_definitions(-DBROTLI_HAVE_LOG2=1)
-  else()
-    add_definitions(-DBROTLI_HAVE_LOG2=0)
-  endif()
-
-  set(CMAKE_REQUIRED_LIBRARIES "${_ORIG_REQ_LIBS}")
-  unset(LOG2_LIBM_RES)
-  unset(_ORIG_REQ_LIBS)
-else()
-  add_definitions(-DBROTLI_HAVE_LOG2=1)
+CHECK_LIBRARY_EXISTS(m log2 "" HAVE_LIB_M)
+if(HAVE_LIB_M)
+  set(LIBM_LIBRARY "m")
 endif()
-unset(LOG2_RES)
 
 set(BROTLI_INCLUDE_DIRS "${CMAKE_CURRENT_SOURCE_DIR}/c/include")
 mark_as_advanced(BROTLI_INCLUDE_DIRS)