Fix implicit fallthrough warning with gcc 9.3.0

../daemon/escape.c: In function ‘escape’:
../daemon/escape.c:49:10: error: this statement may fall through [-Werror=implicit-fallthrough=]
   49 |       if ((flags & LADISH_ESCAPE_FLAG_OTHER) == 0)
      |          ^
../daemon/escape.c:53:5: note: here
   53 |     case '<':               /* invalid attribute value char (XML spec) */
      |     ^~~~

gcc (Gentoo 9.3.0 p2) 9.3.0

daemon/escape.c

@@ -50,6 +50,7 @@ void escape(const char ** src_ptr, char ** dst_ptr, unsigned int flags)
       {
         break;
       }
+      /* fall through intentionally, some compilers are parsing comments */
     case '<':               /* invalid attribute value char (XML spec) */
     case '&':               /* invalid attribute value char (XML spec) */
     case '"':               /* we store attribute values in double quotes - invalid attribute value char (XML spec) */

wscript

@@ -224,6 +224,9 @@ def configure(conf):
     if not conf.env['BUILD_PYLASH']:
        add_cflag(conf, '-Wextra')
        conf.env.append_unique('CXXFLAGS', '-Wno-unused-parameter') # the UNUSED() macro doesnt work for C++
+
+    add_cflag(conf, '-Wimplicit-fallthrough=2')
+
     if conf.env['BUILD_WERROR']:
         add_cflag(conf, '-Werror')
         # for pre gcc-4.4, enable optimizations so use of uninitialized variables gets detected