[llvm] [CMake] Test more things under -D_GNU_SOURCE (PR #116640)

[Raul Tambre via llvm-commits]

https://github.com/tambry created https://github.com/llvm/llvm-project/pull/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.

On my machine:
[CMakeCache_pre.txt](https://github.com/user-attachments/files/17802688/CMakeCache_pre.txt)
[CMakeCache_post.txt](https://github.com/user-attachments/files/17802687/CMakeCache_post.txt)

>From 22745885d325bc50e7bbf6911bb5f9c26f16816d Mon Sep 17 00:00:00 2001
From: Raul Tambre <raul at tambre.ee>
Date: Mon, 18 Nov 2024 17:16:01 +0200
Subject: [PATCH] [CMake] Test more things under -D_GNU_SOURCE

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.
---
 llvm/cmake/config-ix.cmake | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
index 86f2bac7d23e84..b54140c559d366 100644
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -40,6 +40,21 @@ if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
           list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_FILE_OFFSET_BITS=64")
 endif()
 
+check_symbol_exists(__GLIBC__ stdio.h LLVM_USING_GLIBC)
+if(LLVM_USING_GLIBC)
+  # 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.
+  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 +351,6 @@ 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)