Implement #if, #else, #elif, and #endif with tests.

So far the only expression implemented is a single integer literal,
but obviously that's easy to extend. Various things including nesting
are tested here.
diff --git a/tests/glcpp-test b/tests/glcpp-test
index 25685ee..022a236 100755
--- a/tests/glcpp-test
+++ b/tests/glcpp-test
@@ -5,5 +5,5 @@
     ../glcpp < $test > $test.out
     gcc -E $test -o $test.gcc
     grep -v '^#' < $test.gcc > $test.expected
-    diff -u $test.expected $test.out
+    diff -B -u $test.expected $test.out
 done