guix: compile glibc without -werror

Compiling glibc 2.24 and 2.27 with the new GCC 10 results in a number of new warnings,
i.e:
```bash
libc-tls.c: In function ‘__libc_setup_tls’:
libc-tls.c:208:30: error: array subscript 1 is outside the bounds of an interior zero-length array ‘struct dtv_slotinfo[0]’ [-Werror=zero-length-bounds]
  208 |   static_slotinfo.si.slotinfo[1].map = main_map;
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from ../sysdeps/x86_64/ldsodefs.h:54,
                 from ../sysdeps/gnu/ldsodefs.h:46,
                 from ../sysdeps/unix/sysv/linux/ldsodefs.h:25,
                 from libc-tls.c:20:
../sysdeps/generic/ldsodefs.h:398:7: note: while referencing ‘slotinfo’
  398 |     } slotinfo[0];
      |       ^~~~~~~~
```

While we could try and backport all the patches required to fix these up, it would
currently seem easier to disable -Werror, which Guix uses by default when building
glibc.

contrib/guix/manifest.scm

@@ -150,7 +150,7 @@ chain for " target " development."))
                                        #:key
                                        (base-gcc-for-libc gcc-7)
                                        (base-kernel-headers base-linux-kernel-headers)
-                                       (base-libc (make-glibc-without-ssp glibc-2.24))
+                                       (base-libc (make-glibc-without-ssp (make-glibc-without-werror glibc-2.24)))
                                        (base-gcc (make-gcc-rpath-link base-gcc)))
   "Convenience wrapper around MAKE-CROSS-TOOLCHAIN with default values
 desirable for building Bitcoin Core release binaries."
@@ -518,6 +518,9 @@ and endian independent.")
 inspecting signatures in Mach-O binaries.")
       (license license:expat))))
 
+(define (make-glibc-without-werror glibc)
+  (package-with-extra-configure-variable glibc "enable_werror" "no"))
+
 (define-public glibc-2.24
   (package
     (inherit glibc-2.31)
@@ -604,7 +607,7 @@ inspecting signatures in Mach-O binaries.")
                  (list gcc-toolchain-7 "static")
                  (cond ((string-contains target "riscv64-")
                         (make-bitcoin-cross-toolchain target
-                                                      #:base-libc glibc-2.27/bitcoin-patched
+                                                      #:base-libc (make-glibc-without-werror glibc-2.27/bitcoin-patched)
                                                       #:base-kernel-headers base-linux-kernel-headers))
                        (else
                         (make-bitcoin-cross-toolchain target)))))