Fix bugs in configure script

The script expected that the shell supports arrays but only Z shell and
Bash do so. Other shells return syntax errors.

In addition, the script expected the mapnik-settings.env file to exist
and crashed otherwise.

configure

@@ -1,20 +1,32 @@
-#! /bin/sh
+#! /usr/bin/env bash
 
 set -eu
 
 : ${PYTHON:=python}
 
+# Only some shells (Bash and Z shell) support arrays. Therefore,
+# the following code provides an alternative for users calling the script
+# with shells other than Bash or Z shell (e.g. Debian users using Dash).
+THE_SHELL=$(basename $SHELL)
+if [ "$THE_SHELL" != "bash" ] && [ "$THE_SHELL" != "zsh" ]; then
+    if [ -f mapnik-settings.env ]; then
+        echo "WARNING: Reading from mapnik-settings.env is supported with Bash or Z shell only."
+    fi
+    $PYTHON scons/scons.py --implicit-deps-changed configure "$@"
+    exit 0
+fi
+
 # mapnik-settings.env is an optional file to store
 # environment variables that should be used before
 # running tests like PROJ_LIB, GDAL_DATA, and ICU_DATA
 # These do not normally need to be set except when
 # building against binary versions of dependencies like
 # done via bootstrap.sh
+VARS=()
 if [ -f mapnik-settings.env ]; then
     echo "Inheriting from mapnik-settings.env"
     . ./mapnik-settings.env
+    VARS=( $(cat mapnik-settings.env) )
 fi
 
-VARS=( $(cat mapnik-settings.env) )
-
 $PYTHON scons/scons.py --implicit-deps-changed configure ${VARS[*]} "$@"