build: Resurrect running mescc from elsewhere.

After setting-up a buld environment, running something like

   ~/src/mes/wip/pre-inst-env mescc -v -v ~/src/mes/wip/scaffold/main.c

works again.

TODO: Make this less complex.

 * The mescc-lib (and gcc-lib) build directories do not help;
   these were introduced to support `wip-autotools'.
 * We used to opt for short *PATH variables: "." rather than
   /gnu/store/.... or /home/janneke/src/mes/master to avoid
   filling the MES arena. XXX Has this been solved?
 * In the Guix bootstrap, we run module/mescc.scm directly
       $MES -e '(mescc)' module/mescc.scm

   ...so that's why these overrides started to appear in multiple
   places.

* build-aux/pre-inst-env.in (MES_UNINSTALLED): New variable.
* scripts/mescc.in (includedir,libdir): Use it to override these.
* module/mescc/mescc.scm (arch-find): Use it to add <lib>-messc.

build-aux/pre-inst-env.in

@@ -48,6 +48,9 @@ export bindir
 GUIX_PACKAGE_PATH="$abs_top_srcdir/guix${GUIX_PACKAGE_PATH:+:}$GUIX_PACKAGE_PATH"
 export GUIX_PACKAGE_PATH
 
+MES_UNINSTALLED=1
+export MES_UNINSTALLED
+
 LANG=
 LC_ALL=
 

module/mescc/mescc.scm

@@ -267,10 +267,20 @@
 (define* (arch-find options file-name #:key kernel)
   (let* ((srcdest (or (getenv "srcdest") ""))
          (srcdir-lib (string-append srcdest "lib"))
+         (srcdir-mescc-lib (string-append srcdest "mescc-lib"))
+         (libdir (option-ref options 'libdir "lib"))
+         (libdir-mescc (string-append
+                        (dirname (option-ref options 'libdir "lib"))
+                        "/mescc-lib"))
          (arch (string-append (arch-get options) "-mes"))
-         (path (cons* "."
+         (path (append (if (getenv "MES_UNINSTALLED")
+                           (list srcdir-mescc-lib
                                  srcdir-lib
-                      (option-ref options 'libdir "lib")
+                                 libdir-mescc)
+                           '())
+                       (list libdir)
+                       (or (and=> (getenv "LIBRARY_PATH")
+                                  (cut string-split <> #\:)) '())
                        (filter-map (multi-opt 'library-dir) options)))
          (arch-file-name (string-append arch "/" file-name))
          (arch-file-name (if kernel (string-append kernel "/" arch-file-name)

scripts/mescc.in

@@ -37,6 +37,11 @@ export MES_PREFIX
 GUILE_LOAD_PATH=@guile_site_dir@${GUILE_LOAD_PATH+:}${GUILE_LOAD_PATH}
 export GUILE_LOAD_PATH
 
+if test -n "$MES_UNINSTALLED"; then
+    includedir=${includedir-${abs_top_srcdir}/include}
+    libdir=${libdir-${abs_top_builddir}/lib}
+fi
+
 MES=${MES-@prefix@/bin/mes}
 bindir=${bindir-@bindir@}
 includedir=${includedir-@includedir@}