Adjust no-register CXXFlag.

To account for different flag on some versions of Clang.

CMakeLists.txt

@@ -542,10 +542,20 @@ endif (Boost_FOUND)
 # Compiler flags
 
 include (CheckCCompilerFlag)
+include (CheckCXXCompilerFlag)
 check_c_compiler_flag(-Wstringop-truncation have_stringop_truncation)
 if (have_stringop_truncation)
     set(HAVE_STRINGOP_TRUNCATION TRUE)
 endif()
+check_cxx_compiler_flag(-Wno-register have_no_register)
+if (have_no_register)
+  set(REGISTER_CXXFLAG -Wno-register)
+else()
+  check_cxx_compiler_flag(-Wno-deprecated-register have_no_dep_register)
+  if (have_no_dep_register)
+    set(REGISTER_CXXFLAG -Wno-deprecated-register)
+  endif()
+endif()
 add_definitions(-D_GNU_SOURCE)
 
 if (APPLE)
@@ -576,7 +586,7 @@ if (UNIX)
   set( CMAKE_C_FLAGS "-Werror -Wdeclaration-after-statement -Wno-pointer-sign -Wall -Wmissing-prototypes -Wmissing-declarations -Wno-unused ${CMAKE_C_FLAGS}")
   set( CMAKE_C_FLAGS "-Wno-deprecated-declarations -std=gnu11 -Wno-error=parentheses ${CMAKE_C_FLAGS}")
   set( CMAKE_CXX_FLAGS "-Werror -Wall -Wmissing-declarations -Wno-unused -Wno-error=parentheses ${CMAKE_CXX_FLAGS}")
-  set( CMAKE_CXX_FLAGS "-Wno-deprecated-declarations -Wno-register ${CMAKE_CXX_FLAGS}")
+  set( CMAKE_CXX_FLAGS "-Wno-deprecated-declarations ${REGISTER_CXXFLAG} ${CMAKE_CXX_FLAGS}")
   set( CMAKE_C_FLAGS_RELEASE "-O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 ${CMAKE_C_FLAGS}")
 endif (UNIX)
 if (MINGW)