fix(gnu.org/gcc): bottle the libc via --with-sysroot (addresses #8423)

projects/gnu.org/gcc/package.yml

@@ -26,6 +26,19 @@ dependencies:
   gnu.org/mpfr: ">=2.4.0"
   gnu.org/mpc: ">=0.8.0"
   zlib.net: ^1.3
+  linux:
+    # Bottle the libc the compiler targets so end-user `pkgx gcc test.c`
+    # finds stdlib.h + crt*.o + libc.so.6 from the pkgx-integrated bottle
+    # without needing host glibc-devel / libc6-dev. See #8423.
+    # gcc's spec file embeds these paths via --with-sysroot below.
+    #
+    # Pin <2.38: glibc 2.38 introduced C23 symbol redirects
+    # (__isoc23_strtoul, __isoc23_*scanf, …) that the runner's older
+    # glibc lacks, so native gcc's own fixincl.c link fails with
+    # `undefined reference to __isoc23_strtoul`. Cross-builds were
+    # unaffected because they link against the sysroot's libc directly.
+    gnu.org/glibc: "<2.38"
+    kernel.org/linux-headers: "*"
   darwin/x86-64: # since 15.1.0
     libisl.sourceforge.io: ^0
 
@@ -197,6 +210,16 @@ build:
     linux:
       ARGS:
         - --disable-multilib
+        # Install-time sysroot only: the built gcc embeds pkgx glibc
+        # paths so end-user `pkgx gcc test.c` finds stdlib.h + crt*.o.
+        # --with-build-sysroot is intentionally NOT set: configure
+        # tests need to run compiled programs at build time, which
+        # requires the host's dynamic linker (sysroot's ld-linux is
+        # at /opt/.../glibc/v.../lib not /lib so an embedded /lib/
+        # interp path won't resolve).
+        - --with-sysroot={{deps.gnu.org/glibc.prefix}}
+        - --with-native-system-header-dir=/include
+        - --with-glibc-version=2.34
     linux/x86-64:
       LDFLAGS:
         - -pie