build: configure: Oops, substitute @prefix@.

* build-aux/pre-inst-env.in (MES_PREFIX): Respect pre-set value.
* configure (main): Oops, substitute @prefix@.
* configure.sh (subst): Likewise.

build-aux/pre-inst-env.in

@@ -23,7 +23,7 @@ abs_top_srcdir="@abs_top_srcdir@"
 abs_top_builddir="@abs_top_builddir@"
 prefix=${prefix-@prefix@}
 
-MES_PREFIX=${srcdest}mes
+MES_PREFIX=${MES_PREFIX-${srcdest}mes}
 export MES_PREFIX
 
 GUILE_LOAD_COMPILED_PATH="$abs_top_builddir/module${GUILE_LOAD_COMPILED_PATH:+:}$GUILE_LOAD_COMPILED_PATH"

configure

@@ -471,6 +471,7 @@ Some influential environment variables:
                      ("@BASH@" . ,(file-name "bash" deps))
                      ("@GUILE@" . ,(file-name "guile" deps))
                      ("@MES@" . ,(file-name "guile" deps))
+                     ("@prefix@" . ,prefix)
                      ("@guile_site_dir@" . ,guile-site-dir)
                      ("@guile_site_ccache_dir@" . ,guile-site-ccache-dir)
                      ("@VERSION@" . ,VERSION)

configure.sh

@@ -69,6 +69,7 @@ subst () {
     -e s,"@top_builddir@,$top_builddir,"\
     -e s,"@BASH@,$BASH,"\
     -e s,"@GUILE@,$GUILE,"\
+    -e s,"@prefix@,$prefix,"\
     -e s,"@guile_site_dir@,$guile_site_dir,"\
     -e s,"@guile_site_ccache_dir@,$guile_site_ccache_dir,"\
     -e s,"@VERSION@,$VERSION,"\