travis: use 'readlink -f' replacement (for MacOS)

travis/prepare.sh

@@ -1,9 +1,12 @@
 #!/bin/sh
 set -e -x
 
+# Perl version of "readlink -f" (which MacOS does not provide)
+readlinkf() { perl -MCwd -e 'print Cwd::abs_path shift' "$1"; }
+
 SETUP_DIRS=$(echo $SETUP_PATH | tr ":" "\n")
 
-SCRIPTDIR=$(dirname $(readlink -f $0))
+SCRIPTDIR=$(dirname $(readlinkf $0))
 CURDIR="$PWD"
 CACHEDIR="$HOME/.cache"
 SOURCEDIR="$HOME/.source"