optlib2c: escape a double quote char in a description string w backslash

[dreamtigers]

Hello, I'm interested in picking up the elixir branch (elixir-optlib) of this repo, and noticed that said branch has two commits: the first one, is this one I'm doing the pull request for, in which @masatake added the code to escape a double quote char in a string, and the second one, which has the code to parse elixir code.

I noticed that the first commit isn't actually related to Elixir (or at least isn't in a way I could see), so it shouldn't be in the elixir branch, and it's useful enough to be merged into master. That's the reason I took this commit and rebased it to master, fixing a small conflict in the process. So now optlib should be able to parse the double quote char in a description string, and I should be free to hack on elixir-optlib after rebasing it to master.

I'm not 100% sure how to test it, but what I did was

$ ./autogen.sh
$ ./configure
$ make validate-input check

And it ran without any error I could notice.

Any help or thought related to the commit or how to test is well received.

[coveralls]

Coverage remained the same at 85.215% when pulling 352a49a on dreamtigers:optlib2c/doublequote into 8f20f8c on universal-ctags:master.

[masatake]

Hello, I'm interested in picking up the elixir branch (elixir-optlib) of this repo,

Thank you. This change is obviously useful.

We don't test optlib2c directly in our CI environment.
Instead, we run tests code generated by optlib2.c as a parser.

You can test the change with

$  ./misc/optlib2c /tmp/elixir.tags  | grep operator

Without this patch you will see:

		  true, 'o', "operator", "operators (e.g. "defmacro a <<< b")",

This line causes a syntax error as a C language source code.

[dreamtigers]

Oh, I see now. Thank you.