gnu: libgc: Always build with -DUSE_LIBC_PRIVATES.

* gnu/packages/bdw-gc.scm (libgc): Add `arguments' field.
* gnu/packages/make-bootstrap.scm (%guile-static): Remove local `libgc'.

gnu/packages/bdw-gc.scm

@@ -35,7 +35,10 @@
              (base32
               "0phwa5driahnpn79zqff14w9yc8sn3599cxz91m78hqdcpl0mznr"))))
    (build-system gnu-build-system)
-   ;; TODO: Build with -DUSE_LIBC_PRIVATES (see make-bootstrap.scm).
+   (arguments
+    ;; Make it so that we don't rely on /proc.  This is especially useful in
+    ;; an initrd run before /proc is mounted.
+    '(#:configure-flags '("CPPFLAGS=-DUSE_LIBC_PRIVATES")))
    (synopsis "The Boehm-Demers-Weiser conservative garbage collector
 for C and C++")
    (description

gnu/packages/make-bootstrap.scm

@@ -404,13 +404,7 @@
   ;; A statically-linked Guile that is relocatable--i.e., it can search
   ;; .scm and .go files relative to its installation directory, rather
   ;; than in hard-coded configure-time paths.
-  (let* ((libgc (package (inherit libgc)
-                  (arguments
-                   ;; Make it so that we don't rely on /proc.  This is
-                   ;; especially useful in an initrd run before /proc is
-                   ;; mounted.
-                   '(#:configure-flags '("CPPFLAGS=-DUSE_LIBC_PRIVATES")))))
-         (guile (package (inherit guile-2.0)
+  (let* ((guile (package (inherit guile-2.0)
                   (name (string-append (package-name guile-2.0) "-static"))
                   (inputs
                    `(("patch/relocatable"