Fix regression parsing gcc preprocessor linemarkers

These are preprocessor statement in the form:

 # linenum filename flags

Closes #1475

Examples/test-suite/errors/pp_unknowndirective3.i

@@ -0,0 +1,14 @@
+%module xxx
+
+// Testing is_digits detecting gcc linemarkers
+
+// These are valid
+# 1 "/usr/include/stdc-predef.h" 1 3 4
+# 123 "header1.h"
+
+// These are invalid
+#a1 'a.h'
+#1b 'b.h'
+#1c1 'c.h'
+#d1d 'd.h'
+