build: Add `--with-libgcrypt-prefix'; capture the path to libgcrypt.

* configure.ac: Add `--with-libgcrypt-prefix'.  Substitute `LIBGCRYPT'.

* Makefile.am (.scm.go): Pass $(LIBGCRYPT).

* guix/utils.scm (%libgcrypt): New variable.
  (sha256): Use it.

Makefile.am

@@ -75,6 +75,7 @@ CLEANFILES = $(GOBJECTS) *.log
 	$(MKDIR_P) `dirname "$@"`
 	NIX_INSTANTIATE="$(NIX_INSTANTIATE)"					\
 	NIXPKGS="$(NIXPKGS)"							\
+	LIBGCRYPT="$(LIBGCRYPT)"						\
 	GUILE_AUTO_COMPILE=0							\
 	GUILE_LOAD_COMPILED_PATH="$(top_builddir):$$GUILE_LOAD_COMPILED_PATH"	\
 	$(GUILD) compile -L "$(top_srcdir)"					\

configure.ac

@@ -58,6 +58,23 @@ else
    AC_MSG_WARN([Please use `--with-nixpkgs'.])
 fi
 
+AC_ARG_WITH([libgcrypt-prefix],
+  [AS_HELP_STRING([--with-libgcrypt-prefix=DIR], [search for GNU libgcrypt in DIR])],
+  [case "$withval" in
+    yes|no)
+      LIBGCRYPT="libgcrypt"
+      ;;
+    *)
+      LIBGCRYPT="$withval/lib/libgcrypt"
+      ;;
+   esac],
+  [LIBGCRYPT="libgcrypt"])
+
+dnl Library name suitable for `dynamic-link'.
+AC_MSG_CHECKING([for libgcrypt shared library name])
+AC_MSG_RESULT([$LIBGCRYPT])
+AC_SUBST([LIBGCRYPT])
+
 AC_CONFIG_FILES([Makefile
                  po/Makefile.in
 		 guix-build])

guix/utils.scm

@@ -384,14 +384,18 @@ starting from the right of S."
 ;;; Hash.
 ;;;
 
+(define %libgcrypt
+  ;; Name of the libgcrypt shared library.
+  (compile-time-value (or (getenv "LIBGCRYPT") "libgcrypt")))
+
 (define sha256
   (cond
    ((compile-time-value
-     (false-if-exception (dynamic-link "libgcrypt")))
+     (false-if-exception (dynamic-link %libgcrypt)))
     ;; Using libgcrypt.
     (let ((hash   (pointer->procedure void
                                       (dynamic-func "gcry_md_hash_buffer"
-                                                    (dynamic-link "libgcrypt"))
+                                                    (dynamic-link %libgcrypt))
                                       `(,int * * ,size_t)))
           (sha256 8))                           ; GCRY_MD_SHA256, as of 1.5.0
       (lambda (bv)