[CMake] Move dladdr check below _GNU_SOURCE check.

[mysterymath]

Summary

The presence of dladdr on Linux depends on whether or not _GNU_SOURCE is
set.

[phosek]

LGTM

llvm/cmake/config-ix.cmake

if( PURE_WINDOWS )
   check_function_exists(__lshrdi3 HAVE___LSHRDI3)
   check_function_exists(__moddi3 HAVE___MODDI3)
   check_function_exists(__udivdi3 HAVE___UDIVDI3)
   check_function_exists(__umoddi3 HAVE___UMODDI3)
 
   check_function_exists(__main HAVE___MAIN)
   check_function_exists(__cmpdi2 HAVE___CMPDI2)
 endif()
-if( HAVE_DLFCN_H )
-  if( HAVE_LIBDL )
-    list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
-  endif()
-  check_symbol_exists(dlopen dlfcn.h HAVE_DLOPEN)
-  check_symbol_exists(dladdr dlfcn.h HAVE_DLADDR)
-  if( HAVE_LIBDL )
-    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES dl)
-  endif()
-endif()
 
 CHECK_STRUCT_HAS_MEMBER("struct stat" st_mtimespec.tv_nsec
     "sys/types.h;sys/stat.h" HAVE_STRUCT_STAT_ST_MTIMESPEC_TV_NSEC)
 if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
 # The st_mtim.tv_nsec member of a `stat` structure is not reliable on some AIX
 # environments.
   set(HAVE_STRUCT_STAT_ST_MTIM_TV_NSEC 0)
 else()
   CHECK_STRUCT_HAS_MEMBER("struct stat" st_mtim.tv_nsec
       "sys/types.h;sys/stat.h" HAVE_STRUCT_STAT_ST_MTIM_TV_NSEC)
 endif()
 
 check_symbol_exists(__GLIBC__ stdio.h LLVM_USING_GLIBC)
 if( LLVM_USING_GLIBC )
   add_compile_definitions(_GNU_SOURCE)
   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
 endif()
-# This check requires _GNU_SOURCE
+
+# This check requires _GNU_SOURCE.
 if (NOT PURE_WINDOWS)
   if (LLVM_PTHREAD_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES ${LLVM_PTHREAD_LIB})
   endif()
   check_symbol_exists(pthread_getname_np pthread.h HAVE_PTHREAD_GETNAME_NP)
   check_symbol_exists(pthread_setname_np pthread.h HAVE_PTHREAD_SETNAME_NP)
   if (LLVM_PTHREAD_LIB)
     list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES ${LLVM_PTHREAD_LIB})
   endif()
 endif()
 
+# This check requires _GNU_SOURCE.
+if( HAVE_DLFCN_H )
+  if( HAVE_LIBDL )
+    list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
+  endif()
+  check_symbol_exists(dlopen dlfcn.h HAVE_DLOPEN)
+  check_symbol_exists(dladdr dlfcn.h HAVE_DLADDR)
+  if( HAVE_LIBDL )
+    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES dl)
+  endif()
+endif()
+
 # available programs checks
 function(llvm_find_program name)
   string(TOUPPER ${name} NAME)
   string(REGEX REPLACE "\\." "_" NAME ${NAME})
 
   find_program(LLVM_PATH_${NAME} NAMES ${ARGV})
   mark_as_advanced(LLVM_PATH_${NAME})
   if(LLVM_PATH_${NAME})