Add a very simple test for the pre-processor.

Validate desired test cases by ensuring the output of glcpp matches
the output of the gcc preprocessor, (ignoring any lines of the gcc
output beginning with '#').

Only one test case so far with a trivial #define.

tests/glcpp-test

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+for test in *.c; do
+    echo "Testing $test"
+    ../glcpp < $test > $test.out
+    gcc -E $test -o $test.gcc
+    grep -v '^#' < $test.gcc > $test.expected
+    diff -u $test.expected $test.out
+done