gh-132257: Remove -flto-partition=none for Linux LTO builds

Misc/NEWS.d/next/Build/2025-04-08-09-11-32.gh-issue-132257.oZWBV-.rst

@@ -0,0 +1 @@
+Change the default LTO flags on GCC to not pass ``-flto-partition=none``, and allow parallelization of LTO. This has a multiple factor speedup for LTO build times on GCC, with no noticeable loss in performance.

configure.ac

@@ -1914,6 +1914,19 @@ then
   fi
 fi
 
+# Look for parallel LTO support in GCC to speed up builds.
+AC_CACHE_CHECK([If GCC version supports parallel LTO],
+               [ac_cv_gcc_parallel_lto],
+               [AC_PREPROC_IFELSE([AC_LANG_SOURCE([
+                #if defined(__GNUC__) && (__GNUC__ >= 8)
+                  /* GCC parallel LTO supported! */
+                #else
+                  #error "GCC parallel lto unsupported"
+                #endif
+               ], [])],
+               [ac_cv_gcc_parallel_lto=yes],
+               [ac_cv_gcc_parallel_lto=no])])
+
 # Enable LTO flags
 AC_MSG_CHECKING([for --with-lto])
 AC_ARG_WITH([lto],
@@ -2031,7 +2044,12 @@ if test "$Py_LTO" = 'true' ; then
           LTOCFLAGS="-flto"
           ;;
         *)
-          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none"
+        if test $ac_cv_gcc_parallel_lto = yes
+        then
+          LTOFLAGS="-flto=auto -fuse-linker-plugin -ffat-lto-objects"
+        else
+          LTOFLAGS="-flto -fuse-linker-plugin -ffat-lto-objects"
+        fi
           ;;
       esac
       ;;