build: Use a 'guile' executable that doesn't warn about locales.

This gets rid of:

  guile: warning: failed to install locale

when running the 'guix' command on a foreign distro where 'GUIX_LOCPATH'
isn't set.

* Makefile.am (pkglibexec_PROGRAMS, guile_SOURCES, guile_LDADD)
(guile_CFLAGS): New variables.
(install-exec-hook): New target.
(AUX_FILES): Remove 'guile-launcher.c'.
* configure.ac: Use 'GUILE_FLAGS'.

Makefile.am

@@ -47,6 +47,18 @@ scripts/guix: scripts/guix.in Makefile
 	$(AM_V_GEN)$(do_subst) < "$(srcdir)/$@.in" > "$@-t"
 	$(AM_V_at)chmod a+x,a-w "$@-t" && mv -f "$@-t" "$@"
 
+# This is our variant of the 'guile' executable, one that doesn't complain
+# about locales.
+pkglibexec_PROGRAMS = guile
+guile_SOURCES = gnu/packages/aux-files/guile-launcher.c
+guile_LDADD   = $(GUILE_LIBS)
+guile_CFLAGS  = $(GUILE_CFLAGS)
+
+# Have the 'guix' command refer to our 'guile'.
+install-exec-hook:
+	$(SED) -i "$(DESTDIR)$(bindir)/guix"				\
+	       -e 's,^#![[:graph:]]\+,#!$(pkglibexecdir)/guile,g'
+
 nodist_noinst_SCRIPTS =				\
   pre-inst-env					\
   test-env
@@ -330,7 +342,6 @@ dist_noinst_DATA =				\
 
 # Auxiliary files for packages.
 AUX_FILES =						\
-  gnu/packages/aux-files/guile-launcher.c		\
   gnu/packages/aux-files/chromium/master-preferences.json		\
   gnu/packages/aux-files/emacs/guix-emacs.el		\
   gnu/packages/aux-files/linux-libre/5.8-arm.conf       \