gccNGPackages.libatomic: fix the two ways GCC 16 assumes a monorepo build

Both are new in 16 and both come from `libatomic` no longer being buildable
on its own without help.

`configure.ac` now refuses to run with an empty `CFLAGS`: it appends
`-fno-link-libatomic` and needs `AC_PROG_CC`'s conftests to see it, so it
will not let `AC_PROG_CC` supply the default instead.

    configure: error: CFLAGS must be set.

The top level passes them down in a monorepo build; here each library is
configured on its own, so pass what `AC_PROG_CC` would have chosen anyway.

`Makefile.am` then grew an `all-local` hook copying `libatomic.la` into
`../../gcc/`, so a compiler built in the same object tree can link
`-latomic`. There is no such tree here -- the compiler is a finished package
and gets the library through its wrapper -- so the rule just fails:

    install: cannot create regular file '/build/build/../../gcc/libatomic.so.1.2.0'

Drop the hook rather than the rule, to keep the difference from upstream
small.

Neither is caught by checking that patches apply: both are build failures on
a tree that patched cleanly.

Co-authored-by: Ben Siraphob <bensiraphob@gmail.com>
Assisted-by: Claude Code (Claude Opus 5)

pkgs/development/compilers/gcc/ng/16/libatomic/no-gcc-objdir-install.patch

@@ -0,0 +1,26 @@
+From: John Ericson <git@JohnEricson.me>
+Subject: [PATCH] libatomic: Do not install into the toplevel gcc objdir
+
+GCC 16 added an `all-local` hook that copies `libatomic.la` into
+`../../gcc/`, so that the compiler being built in the same object tree can
+link `-latomic`.  There is no such tree here: each runtime library is built
+on its own, against an already-finished compiler, which is given the library
+through the wrapper instead.  The rule just fails:
+
+    install: cannot create regular file '/build/build/../../gcc/libatomic.so.1.2.0'
+
+Drop the hook rather than the rule, so the difference from upstream stays as
+small as possible.
+---
+diff --git a/libatomic/Makefile.am b/libatomic/Makefile.am
+index 49689d451..c389e8cd6 100644
+--- a/libatomic/Makefile.am
++++ b/libatomic/Makefile.am
+@@ -181,7 +181,6 @@ all-multi: $(libatomic_la_LIBADD)
+ # from $gcc_objdir seems to fix the issue.
+ 
+ gcc_objdir = `pwd`/$(MULTIBUILDTOP)../../gcc/
+-all-local: stmp-libatomic
+ stmp-libatomic: libatomic.la
+ 	$(LIBTOOL) --mode=install $(INSTALL_DATA) libatomic.la $(gcc_objdir)$(MULTISUBDIR)/
+ if LIBAT_BUILD_ASNEEDED_SOLINK

pkgs/development/compilers/gcc/ng/common/libatomic/default.nix

@@ -43,22 +43,26 @@ stdenv.mkDerivation (finalAttrs: {
     ''
   );
 
-  # Both are the same upstream change, which is in the GCC 16 release branch:
-  # the backport itself, and the hand-edit of the generated `aclocal.m4` that
-  # `fetchpatch` cannot carry. GCC 16 already includes `../config/gthr.m4`
-  # there.
-  patches = lib.optionals (lib.versionOlder release_version "16") [
-    (fetchpatch {
-      name = "custom-threading-model.patch";
-      url = "https://github.com/gcc-mirror/gcc/commit/e5d853bbe9b05d6a00d98ad236f01937303e40c4.diff";
-      hash = "sha256-U1Eh6ByhmseHQigfHIyO4MlAQB3fECmpPEP/M00DOg0=";
-      includes = [
-        "config/*"
-        "libatomic/configure.ac"
-      ];
-    })
-    (getVersionFile "libatomic/gthr-include.patch")
-  ];
+  patches =
+    lib.optionals (lib.versionAtLeast release_version "16") [
+      (getVersionFile "libatomic/no-gcc-objdir-install.patch")
+    ]
+    # Both are the same upstream change, which is in the GCC 16 release
+    # branch: the backport itself, and the hand-edit of the generated
+    # `aclocal.m4` that `fetchpatch` cannot carry. GCC 16 already includes
+    # `../config/gthr.m4` there.
+    ++ lib.optionals (lib.versionOlder release_version "16") [
+      (fetchpatch {
+        name = "custom-threading-model.patch";
+        url = "https://github.com/gcc-mirror/gcc/commit/e5d853bbe9b05d6a00d98ad236f01937303e40c4.diff";
+        hash = "sha256-U1Eh6ByhmseHQigfHIyO4MlAQB3fECmpPEP/M00DOg0=";
+        includes = [
+          "config/*"
+          "libatomic/configure.ac"
+        ];
+      })
+      (getVersionFile "libatomic/gthr-include.patch")
+    ];
 
   postUnpack = ''
     mkdir -p ./build
@@ -87,6 +91,15 @@ stdenv.mkDerivation (finalAttrs: {
     "--disable-multilib"
   ];
 
+  # GCC 16's `libatomic/configure.ac` refuses to run with an empty `CFLAGS`:
+  # it appends `-fno-link-libatomic` to them and needs `AC_PROG_CC`'s conftests
+  # to see it, so it will not let `AC_PROG_CC` supply the default instead. In
+  # the monorepo build the top level passes them down; each library is
+  # configured on its own here, so pass what `AC_PROG_CC` would have chosen.
+  env = lib.optionalAttrs (lib.versionAtLeast release_version "16") {
+    CFLAGS = "-g -O2";
+  };
+
   preConfigure = ''
     cd "$buildRoot"
     configureScript=$sourceRoot/configure

pkgs/development/compilers/gcc/ng/common/patches.nix

@@ -14,6 +14,15 @@
     }
   ];
 
+  # Only present under `../16`, so a `gitRelease` build -- whose version
+  # directory is `../git` -- has to be pointed here too.
+  "libatomic/no-gcc-objdir-install.patch" = [
+    {
+      after = "16";
+      path = ../16;
+    }
+  ];
+
   # In Git: https://github.com/Ericson2314/gcc/tree/regular-dirs-in-libgcc-15
   "libgcc/force-regular-dirs.patch" = [
     {