Check and enable CLZ intrinsic for decoding.

CMakeLists.txt

 cmake_minimum_required(VERSION 2.6)
 include(CheckIncludeFiles)
 include(TestBigEndian)
+include(CheckCSourceCompiles)
 project(libaec)
 set(libaec_VERSION_MAJOR 0)
 set(libaec_VERSION_MINOR 2)
 set(CMAKE_BUILD_TYPE Release)
 enable_testing()
 
+if(CMAKE_COMPILER_IS_GNUCC)
+  set(CMAKE_C_FLAGS "-Wall -std=gnu99 ${CMAKE_C_FLAGS}")
+endif(CMAKE_COMPILER_IS_GNUCC)
+
 check_include_files(malloc.h HAVE_MALLOC_H)
 check_include_files(stdint.h HAVE_STDINT_H)
 test_big_endian(WORDS_BIGENDIAN)
+check_c_source_compiles(
+  "int main(void) { return __builtin_clzll(1LL); }"
+  HAVE_DECL___BUILTIN_CLZLL
+  )
 
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/config.h.in

config.h.in

 #cmakedefine HAVE_MALLOC_H 1
 #cmakedefine HAVE_STDINT_H 1
 #cmakedefine WORDS_BIGENDIAN 1
+#cmakedefine HAVE_DECL___BUILTIN_CLZLL 1