Correctness and tidy

Test compilation performed with warnings. Author and license added. Test for failing grep on ancient OSX versions. Replaced the test image with something less noisy (which compresses better).

contrib/single_file_decoder/combine.sh

@@ -6,6 +6,8 @@
 # implementation (due to the grep calls) but it runs and works everywhere.
 # 
 # TODO: ROOTS and FOUND as arrays (since they fail on paths with spaces)
+# 
+# Author: Carl Woffenden, Numfum GmbH (released under a CC0 license)
 
 # Common file roots
 ROOTS="./"
@@ -25,6 +27,14 @@ usage() {
   exit 1
 }
 
+# Tests that the grep implementation works as expected (older OSX grep fails)
+test_grep() {
+	if ! echo '#include "foo"' | grep -Eq '^\s*#\s*include\s*".+"'; then
+		echo "Aborting: the grep implementation fails to parse include lines"
+		exit 1
+	fi
+}
+
 # Tests if list $1 has item $2 (returning zero on a match)
 list_has_item() {
   if echo "$1" | grep -Eq "(^|\s*)$2(\$|\s*)"; then
@@ -102,6 +112,7 @@ if [ -n "$1" ]; then
     if [ -n "$DESTN" ]; then
       printf "" > "$DESTN"
     fi
+    test_grep
     add_file $1
   else
     echo "Input file not found: \"$1\""