pack: Add '--relocatable'.

* gnu/packages/aux-files/run-in-namespace.c: New file.
* Makefile.am (AUX_FILES): Add it.
* guix/scripts/pack.scm (<c-compiler>): New record type.
(c-compiler, bootstrap-c-compiler, c-compiler-compiler): New procedures.
(self-contained-tarball): Use
'relative-file-name' for the SOURCE -> TARGET symlink.
(docker-image): Add 'defmod' to please Geiser.
(wrapped-package, map-manifest-entries): New procedures.
(%options, show-help): Add --relocatable.
(guix-pack): Honor it.

tests/guix-pack.sh

@@ -20,9 +20,9 @@
 # Test the `guix pack' command-line utility.
 #
 
-# A network connection is required to build %bootstrap-coreutils&co,
-# which is required to run these tests with the --bootstrap option.
-if ! guile -c '(getaddrinfo "www.gnu.org" "80" AI_NUMERICSERV)' 2> /dev/null; then
+# The bootstrap binaries are needed to run these tests, which usually requires
+# a network connection.
+if ! guix build -q guile-bootstrap; then
     exit 77
 fi
 
@@ -87,6 +87,10 @@ guix pack --dry-run --bootstrap -f docker -S /opt/gnu=/ guile-bootstrap
 # guile-bootstrap is not intended to be cross-compiled.
 guix pack --dry-run --bootstrap --target=arm-unknown-linux-gnueabihf coreutils
 
+# Likewise, 'guix pack -R' requires a full-blown toolchain (because
+# 'glibc-bootstrap' lacks 'libc.a'), hence '--dry-run'.
+guix pack -R --dry-run --bootstrap -S /mybin=bin guile-bootstrap
+
 # Make sure package transformation options are honored.
 mkdir -p "$test_directory"
 drv1="`guix pack -n guile 2>&1 | grep pack.*\.drv`"