as requested by phk.

I found no way to avoid having to escape the backslashes because python,
even for uninterpolated ''' long strings, sees them as line continuations.

The generated output is identical except for two comments (in the code
and the code output by the code).

--- ./lib/libvcc/vcc_fixed_token.o.c	2021-02-02 10:40:03.725244373 +0100
+++ ./lib/libvcc/vcc_fixed_token.c	2021-02-02 10:52:04.921259718 +0100
@@ -834,10 +834,7 @@
 	    "uct vre **, const char *);\nvoid VPI_re_fini(struct vre *);\n"
 	);
 	VSB_cat(sb, "\n");
-
-	/* ../include/vcc_assert.h */
-
-	VSB_cat(sb, "/* ---===### include/vcc_assert.h ###===--- */\n\n");
+	VSB_cat(sb, "/* ---===### vgc asserts (generate.py) ###===--- */\n\n");
 	VSB_cat(sb, "#define assert(e)\t\t\t\t\t\t\t\\\ndo {\t\t\t\t"
 	    "\t\t\t\t\t\\\n\tif (!(e)) {\t\t\t\t\t\t\t\\\n\t\tVPI_Fail(__func"
 	    "__, __FILE__, __LINE__, #e);\t\t\\\n\t}\t\t\t\t\t\t\t\t\\\n"