[CMake] Do all availability checks with -D_GNU_SOURCE (#116640)

When compiling LLVM with -std=c++ instead of -std=gnu we'd fail to detect many newer POSIX functions.
We define it for the whole of LLVM anyway so moving the definition to the top fixes detection of a bunch of these on such setups.
Keeping it at the top also avoids accidentally introducing new dependent checks before it being defined.

llvm/cmake/config-ix.cmake

@@ -40,6 +40,23 @@ if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
           list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_FILE_OFFSET_BITS=64")
 endif()
 
+# Newer POSIX functions aren't available without the appropriate defines.
+# Usually those are set by the use of -std=gnuXX, but one can also use the
+# newer functions with -std=c(++)XX, i.e. without the GNU language extensions.
+# Keep this at the top to make sure we don't add _GNU_SOURCE dependent checks
+# before adding it.
+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")
+
+  # enable 64bit off_t on 32bit systems using glibc
+  if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    add_compile_definitions(_FILE_OFFSET_BITS=64)
+    list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_FILE_OFFSET_BITS=64")
+  endif()
+endif()
+
 # include checks
 check_include_file(dlfcn.h HAVE_DLFCN_H)
 check_include_file(errno.h HAVE_ERRNO_H)
@@ -336,17 +353,6 @@ else()
       "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")
-# enable 64bit off_t on 32bit systems using glibc
-  if (CMAKE_SIZEOF_VOID_P EQUAL 4)
-    add_compile_definitions(_FILE_OFFSET_BITS=64)
-    list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_FILE_OFFSET_BITS=64")
-  endif()
-endif()
-
 # This check requires _GNU_SOURCE.
 if (NOT PURE_WINDOWS)
   if (LLVM_PTHREAD_LIB)