configure: Add --with-download-cache option

To make building without an internet connection easier, a new
./configure option '--with-download-cache' was added that sets
the cache for externals (like pjproject, the codecs and the DPMA),
AND the sounds files.  It can also be specified as an environment
variable named "AST_DOWNLOAD_CACHE".  The existing
'--with-sounds-cache' option / SOUNDS_CACHE_DIR env variable and
'--with-externals-cache' option / EXTERNALS_CACHE_DIR env variable
remain and if specified, will override '--with-downloads-cache'.

Change-Id: I5c3cf15ee61e8fe191b52732303e969854f8d861

build_tools/list_valid_installed_externals

@@ -14,7 +14,11 @@ if [[ -z "${tmpdir}" ]] ; then
 fi
 trap "rm -rf ${tmpdir}" EXIT
 
-sed -r -e "s/^([^ =]+)\s*=\s*(.*)$/\1=\"\2\"/g" ${ASTTOPDIR}/makeopts >${tmpdir}/makeopts
+# We have to pre-process the makeopts file so it will be parsable by bash
+# Surround values with double quotes
+# Convert make  $(or) functions to bash ${name:-value}
+sed -r  -e "s/^([^ =]+)\s*=\s*(.*)$/\1=\"\2\"/g" \
+	-e 's/^([^ =]+)="\$\(or ([^,]*),([^)]+)\)"/_tmp="\2"\n\1="${_tmp:-\3}"/g'  ${ASTTOPDIR}/makeopts >${tmpdir}/makeopts
 source ${tmpdir}/makeopts
 if [[ -z "${ASTMODDIR}" ]] ; then
 	echo "${module_name}: Unable to parse ${ASTTOPDIR}/makeopts."