Support relocable install via DESTDIR

DESTDIR[1] can be now set to specify the root in situations such as when
building a package or installing to a chrooted environment.

If not specified by environment variable (eg. by make) or in mkit.ini,
the default value is governed by MKIT_DEFAULT_DESTDIR, which defaults to
"/".

  [1]: https://www.gnu.org/prep/standards/html_node/DESTDIR.html

src/include/deploy.sh

     fi
 }
 
+get_destdir() {
+    #
+    # Get DESTDIR: first from ENV:DESTDIR, then the default
+    #
+    {
+        ini 1value ENV:DESTDIR | grep . && return
+        echo "$MKIT_DEFAULT_DESTDIR"
+    } \
+      | xargs readlink -m \
+      | grep -m 1 .
+}
+
 get_dst() {
     #
     # Find out target path for src file $2 of group $1
     local grp="$1"
     local root=$(ini 1value "roots:$grp")
     test -n "$root" || die "missing in config.ini: roots:$grp"
-    echo "$root"
+    echo "$destdir/$root"
 }
 
 install() {
     #
     check_env
     local dst group mode src
+    local destdir=$(get_destdir)
     ini values "lists:group" \
       | while read group;
         do
     #
     check_env
     local dst group src
+    local destdir=$(get_destdir)
     ini values "lists:group" \
       | while read group;
         do

src/make.skel

 export MKIT_DIR=${MKIT_DIR:-$(dirname "$0")}
 export MKIT_LOCAL=${MKIT_LOCAL:-.mkit}
 export MKIT_DRY=${MKIT_DRY:-false}
+export MKIT_DEFAULT_DESTDIR=${MKIT_DEFAULT_DESTDIR:-/}
 
 . "$MKIT_DIR/include/mkit.sh" || die "failed to init; check if MKIT_DIR is set properly: $MKIT_DIR"
 

src/mkit.mk

 	@echo build options:
 	@echo "VERSION  = ${VERSION}"
 	@echo "PRERELEASE = ${PRERELEASE}"
+	@echo "DESTDIR   = ${DESTDIR}"
 	@echo "PREFIX   = ${PREFIX}"
 
 build: