tccpp: Allow computed include like 42.h

The include directive needs to be parsed as pp-tokens, not
as token (i.e. no conversion to TOK_STR or TOK_NUM).  Also fix
parsing computed includes using quoted strings.

tests/42test.h

@@ -0,0 +1,13 @@
+/* This file is to test compute #include directives.  It's named so
+   that it starts with a pre-processing number which isn't a valid
+   number (42test.h).  Including this must work.  */
+#ifndef INC42_FIRST
+int have_included_42test_h;
+#define INC42_FIRST
+#elif !defined INC42_SECOND
+#define INC42_SECOND
+int have_included_42test_h_second;
+#else
+#define INC42_THIRD
+int have_included_42test_h_third;
+#endif